For everyone who has not updated to a 5.1.x version. This is a bug fix release and it is up to you if you want to upgrade. If you don’t have any current issues you don’t need to upgrade.
Check all the fixes 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 and Tiger and both can be compiled and installed using the instructions in the documentation set without problems.
Read the documentation on how to ugrade MySQL.