I’ve just upgraded my blog to MT 3.1. The update went fairly smoothly, but I found some pages used the wrong css file name. That may have been there before, so I can’t blame the upgrade for that.
I’ve also found that the “Recent Entries” menu no longer shows all the items. It cuts off anything after the one selected. Looks like a bug in MT, since it happens only with dynamically-generated archives.
The main reason I upgraded was to get the new MT-Blacklist plugin. Comment spam is truly annoying, and this promises to keep it out.