All right, for anyone who wants a workaround for the zip problem until I get around to buying Leopard and making a real fix:

Open up The Unarchiver.app/Contents/Info.plist, and search-and-replace all instances of com.pkware.zip-archive with public.zip-archive.

As suspected, Apple went and changed the UTI for zip files, and this broke all programs that used that UTI. Which apparently was only me because everybody else seems to have understood that UTI handling is totally broken and unreliable.