Ouch.

If twitter is compromised, sites serving twitter js (which is a lot of sites) are potentially compromised too. I've just checked and at least some widgets from twitter are down at present (all?), twimg.com is not responding.

DNS and registrars is a bit of a weak point at present in site security, as once they have that, they can serve users whatever they like. It would be even more damaging and hard to detect if they just tweaked content slightly for a few hours by adjusting some words in stories for some countries rather than hijacking sites.