Two years ago, there was a discussion on asynchronous request processing in OpenRasta: http://groups.google.com/group/openrasta/browse_thread/thread/34a599e21bf65d53
Googleing 'openrasta' and 'async' doesn't yield much more than the above link, and a quick grep through the source code (http://svn.caffeine-it.com/openrasta/trunk/src) did not look promising either.
So is it already possible to prioritize requests (so that IO-bound ones get queued, whereas quick ones will always be processed synchronously), using asp.net?
We've not built anything for it yet, it's technically very possible to do, and quite easily as we already support resumable pipeline. There's just not been enough requests for it to spend the engineering time.