Trying to play mpg or avi file with gst-launch-ext :

gst-launch-ext-0.8 foobar.mpg

output :
No configuration file /home/a/.gst found. You might want to create one.
This is not an error, just a friendly reminder... Check the manpage.

Running command-line 
gst-launch-0.8 filesrc location="foobar.mpg" ! mpegdemux name=demux ! { queue !
mpeg2dec ! ffmpegcolorspace ! xvimagesink } { demux. ! queue ! mad ! osssink }

tube execution in progress

Gstreamer-error : file gstelement.c : line 3007 (gst_element_change_state):
should not be reached.

This is with gstreamer 0.8.5 and gst-plugins 0.8.4 (or 0.8.3).

I don't have any problem to display video stream, using :
gst-launch-0.8 filesrc location="foobar.mpg" ! mpegdemux name=demux ! mpeg2dec !
ffmpegcolorspace ! xvimagesink