There was an article on Wired that covers alot of this, but I wasn’t able to find it.
Anyway, my understanding is that the provision was not that AIM had to be compatible, but that they had to show their source code to all the other IM developers. (I could be wrong about this though)
Apparently this provision isn’t working out, though, because Microsoft and Yahoo and a few other groups have complained that there is no chance that AOL is going to be forced to follow this provision.
So, Microsoft, Yahoo and all the other Instant Messengers (except AIM) are working on developing an IM system that will be compatible with everyone (except AOL) and that it will be open source.