First a ServerSide article I've managed to miss (it's been up since August 2004)  about Java clients.

I *loved* this quote from the introduction:

 Around this time, I began to realize that 80's technology such as HTML will go the way of the VT-100, as more and more people have broadband and larger screens, they want an MTV like experience. That's right, it makes no sense now to develop any HTML application that will be deployed in '05 and operated into '06. People are starting to expect rich content, such as http://january.rr.com/flash . HTML is legacy to me. Let's see.

Also found this description of an open source (as an MS employee I dare not look at the source, but reading about the goals is fine) Java client technology called jThinRich.

Now, the difference I see between these solutions and Avalon is that they are "traditional programmer" oriented.  I differentiate these from the web developer/designer audience that HTML, Flash and other products target.  Avalon's somewhat ambitious goals is to combine these two audiences allowing them to work together and build richer, more powerful apps.