The stateless nature of the web presents some user interface issues. Not least of these is how to handle long running processes most efficiently. Do you keep the user waiting, do you poll, etc? Remember, even if everything is going dandy, normal users like to see something.
This JavaRanch article is a good explication of how to use message driven beans and asynchronous access to data in the web tier to deal with these problems. It leans a bit heavily on WebSphere, but does seem to address some of Dion’s issues about there not being enough use of messaging systems. And it even throws a couple of design patterns in as well.