Twitter has been offering jabber/pub-sub to larger API consumers for a while (most aggregators like tweetscan use it as their interface with twitter)

I agree with the premise that they should be using pub-sub for (most) api clients, but twitter agrees as wel - and have been working towards it (hence large consumers already using it)

I'm not familiar with Tweetscan's specific implementation, but I would assume they're just grabbing the public XML feed, no?

nope - the aggregators/searchers/... run get updates via jabber