I've got to talk with folks like Tim Sneath more often -- with lot's of Avalon team members it's hard to make the rounds regularly; There are many of them and only one of me until we get the other Program Manager hired.  In Build an Avalon Web Browser Application Tim states WBAs are '...perfect for enterprise applications where you don't want to deploy anything to the client (simply browse to http://myserver/myapp.wba) and they'll also be great for really immersive web experiences as Avalon becomes more ubiquitous on the desktop.'

Hey Tim, did you forget developers will be able to create Media Center experiences using Windows Presentation Foundation Web Browser Applications?  If you (or others) need a refresher, check out Lot's of Goodness for Media Center Developers at PDC05.