I downloaded netbeans 6.0m2 as a zip. I only unzipped the archive and started netbeans (I do not like installers which probably pollute the windows registry). It just worked. Also my glassfish-cvs workspace (netbeans 6.0m1) was recognized.
It seems like the the repositories or sessions are not fully compatible:
at org.netbeans.jmi.javamodel.JavaClass$Impl.setName(Unknown Source)
The exception occured only once so it wasn't a big problem. It seems like the netbeans 6.0m2 is a little bit faster. The glassfish-integration still works well and is sufficient for my purposes (log file interceptor and simple JMX-browser).
What's also cool':
Try also: strg + alt + enterWindows can be also undocked - I'm working with multiscreens - its very useful then.