I don't want to take anything away from the testing and bug reports of SME7, but I wanted to share a couple contribs for SME7 - mysql 4.1 and phpmyadmin. I don't want to start any big controversy over this though. Lets just make it very clear from the start that this mysql upgrade will not make your server faster, and you shouldn't do it unless you have an application that has REQUIRES mysql4, not just a desire to have the latest and greatest.
Greg, perhaps it would be worthwhile trying to built some mysql4 RPMs which can sit alongside mysql3. Users could then run mysql3 and mysql4 concurrently. mysql4 would just be an add-on, not used by IMP/twig/twiggy/etc, and can't/won't break standard apps or upgrade paths. You'd need different paths to the binaries, and different db directories and socket path, and different port if TCP access was enabled.