So that wave robots can be hosted with any web host.
Note: For this developer preview release, all Wave robots must be implemented as Google App Engine applications. A future release will introduce the ability to host robots with any web host, at any web address.
I don't think anything has been announced yet. Google is notoriously shady about release planning of their products, so it will probably be "when it's ready".
I don't know anything about timelines - such as when it'll be an "open beta" instead of "invitation beta" - but I'm hoping to port the Robot API from Java to .NET, so that when you can host robots outside AppEngine, you'll be able to do so in ASP.NET.
I've got an empty Google Code project set up for this already, and hope to start porting the Java source code next week, with any luck. You may well be able to start writing robots in .NET before you can start to actually see them in action :) (I'll be able to test internally though - it's not going to be an entirely blind port!) Watch the project to get updates...
It has been announced that the plan is to "start extending the Google Wave preview beyond developers on September 30th".
http://googlewavedev.blogspot.com/2009/07/google-wave-updates-from-todays.html