FINALLY someone graciously took the time to write this plug-in. I have been waiting for this functionality for so long at one point I just stopped looking for it. Came back into my head today and to my delight my search actually served a hit worth following :)
Seems to work with 3.0 already too.
One feature I'm missing though, although it should really be in WP core, is automatic merging of tags. That is, if someone added 'cars' and 'car' to their post, I wouldn't have to go in and do another merge because I had set a filter to merge all 'cars' and 'car' into just 'car' already.