This really is an issue!
I was thinking about restricting view rights to each of the portals default-pages to the selected role and see what's next. But did not yet find time to do atrial. Also this might result just in a page not allowed error...
Alternatively the login-dialog would need to get a redirect based on authenticated role. But actually this would be nice to have out of the box :-)
thx for posting, I'll keep you in the loop
BTW. I'm using JBoss portal 2.6.