Shipped off some quick patches for mmm-mode. Now it works enough to see that it works, but since I don't actually use the languages it has built-in support for (besides RPM specs, but that's broken slightly by Emacs 21) I haven't used it much. My copy of the CWeb book should be arriving today (along with the Computers and Typesetting boxed set!) so I'll be putting together some support for that soon.
mmm-mode looks very promising. It's more sophisticated than multi-mode.el, which is good and bad. It doesn't have to switch between modes all the time like multi-mode, but it can't yet handle cases where you add mode delimiters without using its interface (it uses overlays to keep track).
Other news, updating my resume. Anyone looking for a guy on Long Island?