This would essentially be Push RSS; you could accomplish the same thing via RSS but with polling. A push-based solution would be much cooler though, and would make more sense for notifications or events.
Yes, but that's the easy part, isn't it. Anyone can write a quick polling app based on RSS.
The hard part is (off the top of my head ...):
o Putting together a UI that people can use and will like. Notifications should be quick and easy to set up. They should be noticeable enough to work, but they should not cause problems with existing workflows. And the whole system needs to be easy for the user to understand.
o If a new protocol: get sites all over to use it. If an existing one, figure out how to wrap around it nicely (extract and display the right info, etc.).
o Integrate this with non-RSS stuff, like the things that Growl does.
Brad Fitzpatrick's http://code.google.com/p/pubsubhubbub/ can be used for this. ("Subscribe" to a RSS/Atom feed, it does the polling for you, and gives you callbacks when new entries happen.)
