Yes, support for HTML5 features is all up to WebKit engine. And, as far as i know, Bundyo had a huge problems while trying to update Tear to newest versions of webkit due to damned old GLib. So, chances that Tear will receive support for new html5 features are very low...

Anyway, i don't think that support for EventSource can save some battery or cpu cycles on N8x0. Long-polling techniques (comet) already uses pretty low cpu and traffic. Significant improvements are only possible with carrier support as discussed in whitepaper: http://dev.w3.org/html5/eventsource/#eventsource-push
But for Maemo there is no any carrier-provided push notifications, and TCP-connection (probably with some keep-alives) is still neccessary for receiving events. But this is not much better than long-polling (which also more compatible and cross-browser).