I'm having the same problem. This started to happen to me on two machines after
upgrading to 2.6.3 from 2.6.1. I'm seeing an uniform corruption in most of the
JPEG images I have. The corruption is only in Nautilus thumbnails, loading the
images on Gimp shows them properly.

Image size doesn't seem to affect the corruption. I have found two JPEGs which
are NOT corrupted. One is 80x80, the other is 493x519. Images in between these
sizes (on disk and in dimensions) get corrupted too. Image sizes for the two
images are 1.9 Kb and 32.2 Kb. But that seems not to be a key: One 3.2 Kb image
with the size of 193x88 get's corrupted.

Here are some information on my setup on the other of the machines. I'm using
Gentoo.

Portage 2.0.50-r10 (default-x86-1.4, gcc-3.3.4, glibc-2.3.4.20040808-r0,
2.6.8.1-mm2)
=================================================================
System uname: 2.6.8.1-mm2 i686 Intel(R) Pentium(R) 4 CPU 2.66GHz
Gentoo Base System version 1.5.3
Autoconf: sys-devel/autoconf-2.59-r4
Automake: sys-devel/automake-1.8.5-r1
ACCEPT_KEYWORDS="x86 ~x86"
AUTOCLEAN="yes"
CFLAGS="-O3 -march=pentium4 -funroll-loops -fprefetch-loop-arrays -pipe -fPIC"
CHOST="i686-pc-linux-gnu"
COMPILER="gcc3"
CONFIG_PROTECT="/etc /usr/X11R6/lib/X11/xkb /usr/kde/2/share/config
/usr/kde/3/share/config /usr/lib/mozilla/defaults/pref /usr/share/config
/var/qmail/control"
CONFIG_PROTECT_MASK="/etc/gconf /etc/terminfo /etc/env.d"
CXXFLAGS="-O3 -march=pentium4 -funroll-loops -fprefetch-loop-arrays -pipe -fPIC"
DISTDIR="/usr/portage/distfiles"
FEATURES="autoaddcvs ccache sandbox"
GENTOO_MIRRORS="http://ftp.rhnet.is/pub/gentoo/
http://trumpetti.atm.tut.fi/gentoo/ ftp://trumpetti.atm.tut.fi/gentoo/
http://212.219.247.12/sites/www.ibiblio.org/gentoo/ "
MAKEOPTS="-j2"
PKGDIR="/usr/portage/packages"
PORTAGE_TMPDIR="/var/tmp"
PORTDIR="/usr/portage"
PORTDIR_OVERLAY="/usr/local/portage"
SYNC="rsync://rsync.gentoo.org/gentoo-portage"
USE="X alsa apache2 apm avi berkdb bonobo crypt cups curl emacs encode esd expat
foomaticdb gd gdbm gif gnome gpm gstreamer gtk gtk2 gtkhtml idea imlib jabber
jpeg ldap libg++ libwww mad mikmod mmx motif mozilla moznocompose moznoirc
moznomail mozsvg mpeg ncurses nls oggvorbis opengl oss pam pdflib perl png
postgres postgresql python quicktime readline samba sdl slang spell sse ssl svga
tcpd threads tiff truetype unicode x86 xml xml2 xmms xv zlib"

And these are my current GNOME software versions:

Nautilus  2.6.3-r1
Pango 1.4.1-r1
Glib 2.4.6
GTK+ 2.4.7
LibXML 1.8.17-r2
EEL 2.6.2
Gnome-VFS 2.6.1.1
Gconf 2.6.2
LibGnomeUi 2.6.1.1
LibGnoe 2.6.2
Libart-lgpl 2.3.16
LibBonoboUi 2.6.1
LibBonobo 2.6.2
Orbit 2.10.4
Fam 2.7.0-r1
Gstreamer 0.8.5
Librsvg 2.6.5

And I have jpeg library version 6b-r3.

Created attachment 30912 [details]
Screen capture of thumbnail corruption in Nautilus

This screen capture shows the corruption. I've selected an icon to show how
Nautilus renders the corrupted parts transparent. If you look carefully, all
images are corrupted in the same way. The small maa.jpg image on the screenshot
is one of the two images which have uncorrupted thumbnail in Nautilus.

Seeing this here as well, also on Gentoo with latest Gnome software available in portage.

Portage 2.0.50-r10 (2.6, gcc-3.4.1, glibc-2.3.4.20040808-r0, 2.6.8-gentoo-r2)
===============================================================
==
System uname: 2.6.8-gentoo-r2 i686 AMD Athlon(tm) XP 3000+
Gentoo Base System version 1.5.3
Autoconf: sys-devel/autoconf-2.59-r4
Automake: sys-devel/automake-1.8.5-r1
ACCEPT_KEYWORDS="x86 ~x86"
AUTOCLEAN="yes"
CFLAGS="-march=athlon-xp -O2 -fomit-frame-pointer -pipe"
CHOST="i686-pc-linux-gnu"
COMPILER="gcc3"
CONFIG_PROTECT="/etc /usr/X11R6/lib/X11/xkb /usr/kde/2/share/config /usr/kde/3.3/share/
config:/usr/kde/3.3/env:/usr/kde/3.3/shutdown /usr/kde/3/share/config /usr/lib/mozilla/defaults/
pref /usr/share/config /usr/share/texmf/dvipdfm/config/ /usr/share/texmf/dvips/config/ /usr/
share/texmf/tex/generic/config/ /usr/share/texmf/tex/platex/config/ /usr/share/texmf/xdvi/ /var/
qmail/control"
CONFIG_PROTECT_MASK="/etc/gconf /etc/terminfo /etc/env.d"
CXXFLAGS="-O2 -pipe"
DISTDIR="/usr/portage/distfiles"
FEATURES="autoaddcvs ccache sandbox sfperms"
GENTOO_MIRRORS="http://gentoo.osuosl.org http://distro.ibiblio.org/pub/Linux/distributions/
gentoo"
MAKEOPTS="-j2"
PKGDIR="/usr/portage/packages"
PORTAGE_TMPDIR="/var/tmp"
PORTDIR="/usr/portage"
PORTDIR_OVERLAY="/usr/local/portage"
SYNC="rsync://rsync.gentoo.org/gentoo-portage"
USE="3dnow 3dnowext X alsa apm arts avi berkdb bindist cjk crypt cups dba dvd encode esd f77 fam 
foomaticdb gd gdbm gif gnome gpm gtk gtk2 imap imlib java jpeg kde libg++ libwww mad mikmod 
mmx mmxext motif mozilla mpeg mysql ncurses nls nptl nvidia oggvorbis opengl oss pam pdflib perl 
png ppds python qt quicktime readline samba sdl slang spell sse ssl stencil-buffer svga tcpd tiff 
truetype usb v4l wxwindows x86 xml xml2 xmms xv xvid zlib"

Package versions are same as reporter above.

I'm having the same problem also, also using the same software posted above.
Here's my emerge --info...

Portage 2.0.50-r9 (default-x86-1.4, gcc-3.3.3, glibc-2.3.3.20040420-r1, 2.6.9-rc1)
=================================================================
System uname: 2.6.9-rc1 i686 Mobile Intel(R) Pentium(R) 4     CPU 3.06GHz
Gentoo Base System version 1.4.16
Autoconf: sys-devel/autoconf-2.59-r4
Automake: sys-devel/automake-1.8.5-r1
ACCEPT_KEYWORDS="x86"
AUTOCLEAN="yes"
CFLAGS="-march=pentium4 -O3 -pipe -fomit-frame-pointer"
CHOST="i686-pc-linux-gnu"
COMPILER="gcc3"
CONFIG_PROTECT="/etc /usr/X11R6/lib/X11/xkb /usr/kde/2/share/config
/usr/kde/3/share/config /usr/lib/mozilla/defaults/pref /usr/share/config
/var/qmail/control"
CONFIG_PROTECT_MASK="/etc/gconf /etc/terminfo /etc/env.d"
CXXFLAGS="-march=pentium4 -O3 -pipe -fomit-frame-pointer"
DISTDIR="/usr/portage/distfiles"
FEATURES="autoaddcvs ccache sandbox"
GENTOO_MIRRORS="http://open-systems.ufl.edu/mirrors/gentoo"
MAKEOPTS="-j2"
PKGDIR="/usr/portage/packages"
PORTAGE_TMPDIR="/var/tmp"
PORTDIR="/usr/portage"
PORTDIR_OVERLAY="/usr/local/portage"
SYNC="rsync://rsync.gentoo.org/gentoo-portage"
USE="X alsa apm avi berkdb cdr crypt cups directfb encode esd foomaticdb gdbm
gif gnome gpm gtk gtk2 imlib java jpeg libg++ libwww mad mikmod motif mozilla
moznocompose moznoirc moznomail mpeg ncurses network nls oggvorbis opengl oss
pam pdflib perl png python quicktime readline samba sdl slang spell ssl svga
tcpd truetype x86 xml2 xv zlib"

Created attachment 30947 [details]
Here's a shot with the thumbnail distortion

I'm also having the same problems with .png image corruption for larger images.
So it isn't just the jpeg format.

Looking closer at the problem, I can confirm PNG images are corrupted too, like
Craig noticed. But the problem is not with large images! I have a 16x16 pixels
and 833 bytes PNG which has corrupted thumbnail. Along with PNG, PNM images are
affected by the corruption.

TIFF images seem to work perfectly. I looked at all the TIFF files I have and
none of them were corrupted. I also created two TIFF versions from a JPEG which
had a corrupted thumbnail. First was without compression, and the second was
with JPEG compression (could not test LZW because Gimp refused to save with it
selected). The thumbnails for the TIFF images were working perfectly.

This appears to be a bug in gtk+ 2.4.7 ( and maybe 2.4.8 as well ).  The
corruption of thumbs ceased after replacing gtk+ with 2.4.9.

From GTK+ 2.4.9 change log:

"Fix a problem with GDK_INTERP_NEAREST scaling which caused
Nautilus thumbnails to be misdrawn [Christoph Fergeau]"

*** Bug 151107 has been marked as a duplicate of this bug. ***

Yep, confirming this bug is fixed.