Wow, using JBoss portal for something heavily used? That is what I call optimism. :-)
If you asking "can the JBoss 4 **APPSERVER** scale", the answer is yes if you wrote your application to scale. In other words, JBoss thankfully doesn't stop you from having a scalable application. That doesn't mean the developer actually wrote and configured an application to be scalable.
Last time I tried the portal app was about 4 months ago, and frankly I couldn't imagine having 500 users on it. I found it sluggish and fragile, even with just lil' ol' me on it. Things may have changed, definitely, but you'll need to do your own
testing to be sure.