The AVC profile will be a radical departure from the other profiles. I would rather see a fork into a new project (or even start from scratch with the reference source and optimize from there).
It's just that almost all the core components - the transform, quantization method, motion compensation, block sizes, frame referencing, vlc codes, block storage syntax, etc. - are different compared to the existing MPEG-4 profiles. It could be implemented inside XviD, but there'd be such small amounts of shared code it would cause a lot of headaches - about the only functions you could share between AVC and the rest would be sad16/sad8 and colorspace conversions!
Not that I wouldn't like to see it done
I know Isibaar has been keeping an eye on the development of the standard. I haven't looked too closely at it actually, so it might have more shared parts than I'm thinking.
-h