Actually, there is no Java 5.1 and never will be.
Instead, Java 1.5.0_04 has been released today. So if you were waiting for the dot 1 release (because you just don’t use dot 0 releases), with over 350 bugs fixed since 1.5.0, this is the release you’ve been waiting for.
Go get it! (and use it with the NetBeans Profiler, Milestone 7 was just released.)