Sun have announced the availablity of the GlassFish Tools Bundle for Eclipse. Both GlassFish v2.1 and v3 Prelude are bundled and pre-configured with the build. The bundle is based on Eclipse IDE 3.4.1 Java EE.
John Clingan has also written about this release.
To preempt the tinfoil hats, GlassFish will continue to ship with the award winning NetBeans IDE. In fact, check out this video Adam Bien's presentation at Community One East on the NetBeans 6.7 with GlassFish v3 Prelude. 3 slides and a ton of code