Heroku (YC08) has been around longer than #22 (http://ycombinator.com/ideas.html), but pg's item on fundable ideas might hint at some of his vision regarding abstracted platforms:
Don't make it feel like a database. That frightens people. The question to ask is: how much can I let people do without defining structure? You want the database equivalent of a language that makes its easy to keep data in linked lists.
I realize that Derby also helps build real-time applications, but I haven't heard anything about Derby being in YC. Has there been a public announcement that they are?
If we're just listing frameworks that help build real-time systems, Pusher would be another example.
Don't make it feel like a database. That frightens people. The question to ask is: how much can I let people do without defining structure? You want the database equivalent of a language that makes its easy to keep data in linked lists.