In order to do http load balancing with glassfish - you'll have to use the Sun Web Server - not Apache. Some people are using ajp as a connector port by hacking the domain.xml but I don't believe that a support configuration out of the box.
It would be nice if Sun actually supported the AJP out of the box so that one can use the latest load balancing modules that works with AJP on the latest apache servers.
I'm not sure what you mean about clustering but in the JEE world - it usually means two JEE servers in clustered mode sharing HTTP Sessions, JNDI, along with JMS clustering.
It's probably wiser to split out your environment so you have separate / distinct tiers.
One for DEV, one for QA, one for UAT (user acceptance testing), and one for PRODUCTION.
The first three tiers can easily be done in one box via XEN. Production should be segregated out to a different physical box - definitely wouldn't be wise mixing those tiers with production as buggy code usually has the habit of causing resource failures.
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