I'm skeptical of this happening. Right now it's easy to upgrade while the community is small and people download their client from one source.
But once everybody is running different flavours of Bitcoin, network-wide upgrades are not a solution.
I share your concern, that's why I proposed this: http://bitcointalk.org/index.php?topic=1865.0
It would still need one backward incompatible change, but just one. Gavin once showed himself open to the idea, if somebody manages to implement it safely. Now we "only" need somebody good enough in C++ willing to give it a try...