For the UI, I wanted to have two main interfaces: html (+ friends) for human consumption and some webservices (SOAP, etc.) for offering the data to external apps (instead of allowing them to tap into the DB directly)
The number of concurrent users souldn't go much above 200 - 500.
The first question you would want to ask yourself is "What parts of the J2EE API do I want to use, and why?" To get the polling interval going, I would use something like a Servlet or an MBean to do it, which would fit well with JBoss.
Based on the UI requirements that you've mentioned here, I would still use a JBoss-based implementation. However, I might look into distributing the workload across a few dozen nodes. The problem that you might face is the large number of connections that you want to have. Depending on how much traffic you're generating, the problem might not be with the server but with the network hardware. Keep that in mind as well when you do your initial performance testing, and think about how you could split the work across a few different servers.