I can see where that may be an issue, given my thin understanding of versioning in Apple's document model. IIRC, you can declare a document versioned or not, but there's no turning it on or off after creation. Can anyone confirm this?
I think adding versioning to the mix as an option is interesting for those who use TextMate as a ToDo editor, for simple one-offs outside of larger projects, and so on. I can see myself using it in those situations, and continuing to use hg and git for my significant projects, both source and document (LaTeX mostly) based.
If TM were to then provide that diff highlighting on top of the versioning, that would, I think, be a rather spiffy feature set. I have a suspicion that integrating Apple's versioning into the current SCM handling model of TM would be a significant amount of trouble, but it would be rather slick if possible.