633k has a nice little program that appears to be what I've been looking for, that is, something that will go through my iTunes library and fix all my genre tags from a trusted source (ie: Last.fm).

Anyone have anything else that might do the same?

(Turns out this isn't all that great, bugs in the program (redraw stuff) and it doesn't seem to do all-at-once, you have to tag each artist or each song :( ).