Apple rarely stream their keynotes. It’s a rare event, and usually involves a tie-in to their ecosystem (e.g. you must use Safari on OSX to view the keynote, etc.). Apple have confirmed that their event later today will be streamed for those who own an Apple TV.
The confirmation comes in the form of an official “Apple events” app on the Apple TV store. There is still a chance the event will be streamed via browser, but nothing confirmed has come in.
The fact that it’s being streamed anywhere means that others can easily re-broadcast on other platforms – so everyone will likely get a shot at viewing the keynote.
The keynote, by the way, is likely to be an announcement of the iPad Mini and 13-inch MacBook Pro with Retina. There’s probably more to come (including the official launch of the new iTunes), but the guts of it is likely to focus on the smaller iPad form-factor, and what that means for users, content creators and, I imagine especially, education.