The list of benefits and features of GlassFish v3 Prelude is long and impressive.
One thing I did not see on the list but is present and working very well is LDAP support.
In the OpenDS project we document "How to secure web applications deployed in GlassFish with OpenDS". The example is written for GlassFish v2 but continues to work with GlassFish v3 Prelude.
You can also run OpenDS LDAP directory server, embedded in a Web Application deployed in GlassFish
Or deploy OpenDS DSML Gateway in GlassFish
Edit on Nov 7th.
I forgot to mention that there will be a chapter dedicated to OpenDS on the coming book : GlassFish in Action (and it looks like the chapter is already available in the Manning Early Access Program).