I have a sequence that will decompose captured clips just fine but if I try and decompose imports the sequence gives me this error. 

Assertion failed: srcMob

File: /Snapshots/releng.relengintel4_1230513104/vobs/ppg/MCTop/ProjectFiles

/../../MediaComposer/Media_Composer/

Decomposer.c,  Line: 133

Any idea on what could be causing this error?

based on the language in the error, I'd say that one or more import is corrupted.  Can you simply delete the associated media on the clip and import the clip again?

I had to go through and sub out piece by piece to isolate about 6 clips that were "corrupt".  Although they play back fine they just wouldn't decompose.  These were actually transcoded clips at 10:1m from P2 cards so I just ended up going to the full res media and overcutting.  A lengthy process but problem solved.  Thanks for the advice!  What language in the error pointed you to a corrupt import?  I need to learn to troubleshoot these avid error messages when they come up,

vobs...  Video objects.  Avid speak can be tricky.  You get lucky sometimes