JJG over at Adaptive Path has a nice article on rich clients in their current incarnation of asynchronous JavaScript and XML. He calls this Ajax. It's good and worth reading if you're not already familiar. I have one small problem with it:

By defining an architectural style as it's building materials and not with it's underlying philosophy, it will quickly become obsolete. In fact, it's already obsolete: Google Suggest doesn't use XML for data interchange; just JavaScript arrays. Gmail also doesn't use XML and it doesn't use standards-based markup for presentation. Are they no longer Ajax applications? Of course they are.

What's amazing here isn't that there are rich clients on the web that use an asynchronous tier. What's amazing is that they aren't built using proprietary technology; they're not Flash, Java, or ActiveX.

Because they're not proprietary, we can, as JJG says, use the techniques we already know and are well deployed. This is also true for scaling Ajax applications; we can use the stuff we already know; Etags, Range, If-None-Match, etc.