There appears to be a few 'flavors' of the cab file with differing contents.
I'm using a cab that was recompiled to include a mp4/aac plugin
and seems to play mp4/aac and mp4/avi's with DivX/XviD encoding
and .flv's from youtube fine.
Take a look at
http://forum.xda-developers.com/showthread.php?t=380387
that has links to download the modified file.
Changelog
recomp-02
Added MPEG4.PLG from source found in TCPMP 0.66
FFMPEG updated
recomp-03 - 27/5/08
Installs and runs on WM2003 devices
Detects Kaiser, allows 'Direct' video output
Source zip now has ASM files
Small optimizations