I found Factor CSS – complete with source code, but I think it does way more than I’d need. I really just want to combine CSS blocks that have the same selectors. I’ll check out the source code and see if it can be converted to something usable as a TextMate bundle. That is, unless someone else manages to get to it before me.
EDIT: Even better – here’s a list of web-based tools for checking/formatting/optimizing css.