That's a bad idea, because you end up piggybacking on top of another service, unless you do something like.. individual users run their own jabber servers.
Or you could provide a jabber server that uses your web app's authentication mechanism so users just need to add another IM account to the IM account they are already using. Or things like growl could take an jabber server as a configuration option to receive notifications from. Desktop notification is mainly just a reformatting of the UI on technologies that already exist, are in use, and are pervasive.
You could also run a laconi.ca microblogging server, here's a list: http://laconi.ca/trac/wiki/ListOfServers and any event is sent out from one account from which all users are subscribed.
