JBoss supports clustering, so it should be possible to connect multiple servers. See this: http://www.jboss.org/jbossclustering
But I fear that performance could be too bad for a MMORPG, because JBoss is targeted for "very high level" communication technics, where the communication overhead could kill you... For a MMORPG, you would probably better use TCP/IP communication to achieve best performance and serve thousands of users...
Do you have a MMORPG written in Java? One that requires an app server to run? If so, then JBoss AS could work.
Or are you planning to write one in Java? Besides the concerns that Wolfgang mentioned, your biggest issue will be garbage collection pauses. You should use one of the low-pause collector such as CMS or the new G1 collector. And that means you should use machines with multiple cores (4 at the minimum) and large amounts of RAM with huge heaps (2 GB minimum). That is, if you want more than a few dozen concurrent players online.
But forget about the server side for a minute. On the client side you will also have to provide the UI for the player. If that is written in Java also then you will really have concerns regarding garbage collection. You cannot automatcially assume that the player will be running a on a large machine (4 cores, >=4GB RAM, 64-bot OS) and thus it might not be practical to run the CMS or G1 collectors, in which case the player could experience the game freezing for several seconds every few minutes when the garbage collector runs.
Generally Java may not be the best language for a heavily graphical game. Obviously it can be done (http://www.java-gaming.org/). If you write the client in C++, though, you may be able to utilize Java as the server side code and definitely use a JBoss cluster to server out the server data.
It can be done either way, so it depends on exactly what your specs are. With online gaming, you are going to want to consider performance, which is definitely a topic when you write in Java as opposed to C++.
Take a look at Java Gaming and see if your spec seems reasonable in Java, or if you may want to consider a different language for the client or server side.
Ok thank you everyone for your advices !
In fact, I want to learn the Objective C language so I planed to develop a MMO game on Iphone (a little bit like Yslandia), thus the client side will be written in Objective C.
But you're right, garbage collector and memory would be a too important issue for the MMO game I plan to develop.
So, do you have any idea of an open source server that should better correspond to my needs ? (Better if it supports clustering )