I found myself asking the same question. Not sure how heroku's system is much different. Running a web application is running a web application, right? Saying you need "push" is a lousy excuse IMO. Its a "the backend is the backend, right?" question?
Unless I'm missing something, offering sinatra or something lighter would make sense... but it still seems like PaaS to me.
At first I thought they were in deep trouble with this hitting the front page mere hours after the iCloud announcement. Then I looked at what the actually offer, and it looks like they will be able to compete. They have social, notifications, cross-platform-ness, and analytics in addition to just storage sync. Good luck, I may become a user!
Looks really good in principle, although I think they're offering too much for a first version. The app backend alone is compelling enough for v1.0 IMO.