Kellan has a nice addition to the Twitter (and social software in general) scaling discussion. Interestingly, he's giving a talk at OSCON entitled "Beyond REST? Building Data Services with XMPP PubSub."
XMPP PubSub is (part of) what makes collaboration work in Sugar as well, and that's got its own scaling issues on a school server. It isn't the "how do we handle hundreds of thousands of users with a rack of blades (or whatever)" problem, but the "how do we maintain very chatty collaboration between hundreds of kids on an inexpensive school server that's also doing 10 other things at the same time" problem. It seems to me that OLPC probably needs a heavily optimized server to do what they need, and finding someone to do that won't be easy. It is one reason I think Sugar needs a couple years and not a few months of love.
XMPP Pubsub is also the best route to a modern, open source SIF implementation (not that I expect to ever see one).
Regardless... more attention to XMPP PubSub will be a good thing for everyone.