Gentoo Portage now contains new MySQL ebuilds that use slots. Slots are portage’s mechanism to allow different versions of a package to be installed at the same time. Without slots, you’re restricted to one version. The best known examples of slotted packages are KDE and the various kernel packages. To summarise all of this: with the new ebuilds you can have e.g. MySQL 4.1, 5.0 and 5.1 installed at the same time.
Moving to the new ebuilds, which are no longer hard masked but are still in the unstable branch (~x86 in my case), does require a bit of work. But don’t panic! The Gentoo devs have written a migration guide to ease the switch for you.
I’m currently in the process of switching to the 5.0.18 slotted version…The switch to 5.0.18-r30 is now complete