Sun just released jdk1.6.0_01. See the Release Notes.
I will update my personal development boxes to it. But for my current project, I'll continue using Java 4.
Are you on one of those projects where you are using Java 4 and see no sign that it will be upgraded soon? (By soon, I mean within the next 18 months.)
For a while earlier in the year, I heard something like "Java 5 will be allowed." And then reality hit with "Yeah, you can use the Java 5 JVM but you should not use any of the Java 5 language features."