There’s a number of interesting synergies that come to mind (I’m not saying that a merger would or wouldn’t work – honestly, who can tell) and they’re not necessarily AdWords related.

The whole point of Twitter is that it reflects, in many ways, the zeitgeist. I’ve seen breaking news reported on the site more than an hour before it hit the big news sites like CNN. A company like Google could get all kinds of useful information out of scanning keywords on Twitter and cross-referencing them. They could then feed that information into their news site, or even into search listings, in order to make them reflect what is going on in the world. It would probably be hard to prevent that from being “game-able”, but it might be interesting…