For everyone who has upgraded to a 5.1.x version. This is a bug fix release and it is up to you if you want to upgrade. There are no apparent security issues solved in this release.
Check all the fixes and changes that are listed on the release notes to see what issues are resolved and if you are affected.
I’ve compiled this version and did some simple tests on Leopard (not on Tiger yet) and both can be compiled and installed using the instructions in the documentation set without problems.
Read the documentation on how to ugrade MySQL.