I think your analysis of notifications is good, but I'm not sure I agree with your conclusion about a centralized system. I go to Facebook when I want a break from thinking; I go to Twitter when I want some quick inspiration and maybe a link to a thought provoking article; I go to GReader when I want to wade through several articles (or to see if Rands has posted again). HTC has a unified Twitter, Facebook, and Flickr. I haven't turned it on because I have no desire to see these things together.

Perhaps my use case is unique. Maybe Steve or someone at Google or someone at a startup can show me why I need notifications for these services combined, but for now, I have no problem having my notifications separate.