Hm, I didn't think about ads. That could really be a problem. I use Adblock Plus and NoScript plugins in Firefox, so I don't see a lot of these, and I'm sure they could also help to filter unwanted push.
So the protocol should allow the client to filter the push list *before* it's actually sent. A simple handshake would help:
server: I want to send x.html, ad.html, pic.jpg, ad.swf
client: Just send x.html, pic.jpg
And so the filter works before the push. After all, we want things faster, so filtering things the client won't see anyway before it's pushed seems necessary.