I don't see notifications supplanting Server Sent Events. SSE gives developers a lot more control over the stream since it's connected directly to your server, and you don't have to bother the users with permission dialogs. You'd use push notifications if the user doesn't have your site open, but otherwise SSE gives you more power.
(I'm working on push notifications.)
As for B2G, I think I just got excited about it because notifications was the bit I was interested in seeing work out.
They're very different notifications. SSE is only useful if the user happens to have the website open. This new proposal is a replacement for emails - it lets you send asynchronous notifications to the user that he may receive even if he's not on your website and even if he was offline for a while.
Does anyone know why they're pushing this instead of plain old Server Sent Events (something w3c is already working on for servers to push to browsers)?