Vince has more on the NetBeans/GlassFish integration.
If you’re on the GlassFish 1 branch (Sun AS 9.0), stick to NetBeans 5.5 for now. GlassFish v2 should be in beta by February and final before JavaOne (probably an indication for the NetBeans 5.5.1 schedule).
update : Note there’s an Enterprise Pack to go with it in the same download section.
update 2 : You can also use NetBeans 6.0 starting with Milestone 6.