Start looking at
theserverside.com,
where they regularly have updates on new version of app servers, comparisons, etc. There was recently one comparing WebSphere 5 to the alternatives.
As far as your skills go, I'd say learn whatever the market is demanding. Sadly, for most businesses, commercial app servers are the name of the game, so I suspect you're unlikely to get a job which requires JBoss experience. That said, using JBoss is a great way to put together your own apps and learn the basic J2EE concepts, and then you can try porting one to a commercial product to get a feel for the differences. Then again, since JBoss currently charges for their main docs and the commercial players don't, maybe this isn't entirely true.
Anyway, personally, I'm not a big fan of WebSphere <=4.x. I hear 5 is better, but it's probably goign to take time to get adopted. Oracle 9iAS (in OC4J form not EE form) is quick and easy to install and run, but doesn't have particularly good validation and error messages for EJBs (at least by default). WebLogic has some faults (pretty bad install and startup time and often iffy redeployment), but is generally pretty good about at least giving you good hints if not clear error messages. That's probably important while you're learning.
Anyway, you'd probably be well served to pick up a good book on EJB and a good book on your preferred app server, if your budget can support it (
Bookpool, perhaps?). I suppose I could recommend a good WebLogic book, if you twist my arm...
Then put together some app you find interesting for personal use -- a database to hold your CD collection or recipes or whatever.
[ February 27, 2003: Message edited by: Aaron Mulder ]