If you are looking for something you want to use right away, check out JRebel. Mechanisms for hot swapping classes is still very premature in the JEE and I'd recommend WAR/EAR deployments in any environment other than development. I find hot swapping more useful for development as it gives me a faster dev/deploy cycle similar to PHP.
Actually the project offers a free patch for the VM to download that you can use today.
But I agree that JRebel is the more robust solution right now.