I'm compiling from source on Debian Etch, which has
   /usr/lib/libecal-1.2.so -> libecal-1.2.so.6.0.0

During compilation libedata-cal-1.2.so is linked against the more
recent libecal-1.2.so.7 which was compiled from source as part of
the same "make". So far, so good:

evolution-data-server/calendar/libedata-cal$ readelf -a .libs/libedata-cal-1.2.so.6.0.2 | grep libecal
 0x00000001 (NEEDED)                     Shared library: [libecal-1.2.so.7]

During installation libtool decides to relink the shared object and
at that stage picks up libecal from /usr/lib first:

libtool: install: warning: relinking `libedata-cal-1.2.la'
(cd /tmp/runtests/head/tmp/evolutiontrunk-build/obj/evolution-data-server/calendar/libedata-cal; /bin/sh ../../libtool  --tag=CC --mode=relink ccache gcc -g -Wall -Wmissing-prototypes -Wno-sign-compare -Wno-pointer-sign -L/lib -L/home/patrick/evo-svn/gtk+2.0/lib -o libedata-cal-1.2.la -rpath /tmp/runtests/head/tmp/evolutiontrunk-result/lib -version-info 6:2:0 Evolution-DataServer-Calendar-common.lo Evolution-DataServer-Calendar-skels.lo Evolution-DataServer-Calendar-stubs.lo e-cal-backend.lo e-cal-backend-cache.lo e-cal-backend-factory.lo e-cal-backend-sexp.lo e-cal-backend-sync.lo e-cal-backend-util.lo e-data-cal.lo e-data-cal-factory.lo e-data-cal-view.lo -pthread -L/home/patrick/evo-svn/gtk+2.0//lib -lxml2 -lgnome-2 -lpopt -lbonobo-2 -lbonobo-activation -lgnomevfs-2 -lgconf-2 -lgmodule-2.0 -ldl -lORBit-2 -lgthread-2.0 -lrt -lgobject-2.0 -lglib-2.0 ../../calendar/libecal/libecal-1.2.la ../../libedataserver/libedataserver-1.2.la )

The result is a broken libedata-cal:
evolution-data-server/calendar/libedata-cal$ readelf -a .libs/libedata-cal-1.2.so.6.0.2T | grep libecal
 0x00000001 (NEEDED)                     Shared library: [libecal-1.2.so.6]

The libtool call above looks wrong to me: libecal-1.2.la and 
libedataserver-1.2.la are specified after some of the additional
libraries they typically depend upon, like libglib.

Changing the order of libedata_cal_1_2_la_LIBADD so that the .la libs are
mentioned first fixed the problem for me. I don't think this can break anything
because .la files are self-contained and thus ordering shouldn't be relevant,
but please test and apply if you agree.

I checked my install .so files (including the extensions) and found
that evolution-data-server/calendar/backends/caldav has the same problem,
while none of the others that I build has. Incidentally, caldav also
lists the $(EVOLUTION_CALENDAR_LIBS) while the other backends don't.
This further confirms that listing the .la files first is probably
the right thing to do.

Index: evolution-data-server/calendar/libedata-cal/Makefile.am
===================================================================
--- evolution-data-server/calendar/libedata-cal/Makefile.am     (Revision 8531)
+++ evolution-data-server/calendar/libedata-cal/Makefile.am     (Arbeitskopie)
@@ -46,9 +46,9 @@
        e-data-cal-view.c
 
 libedata_cal_1_2_la_LIBADD =                                   \
-       $(EVOLUTION_CALENDAR_LIBS)                              \
        $(top_builddir)/calendar/libecal/libecal-1.2.la         \
-       $(top_builddir)/libedataserver/libedataserver-1.2.la
+       $(top_builddir)/libedataserver/libedataserver-1.2.la    \
+       $(EVOLUTION_CALENDAR_LIBS)