Say Google buys Twitter. It was always something that made sense, and now it's gone from "what if" to rumor to late stage negotiations in less than a day. Let's assume it happens and take a peek into what it means for marketers.
Search: Twitter will not become Googleized. With its YouTube acquisition Google showed itself to be a good integrator of other companies. However, it makes sense that Google will clean up the Twitter interface to be more friendly to contextual ads, people search, and keyword search. Twitter will also become more trackable on its own, as well as within Google. And marketers will be intrigued at the possibilities just at the time that search was topping out.
Microsoft, Yahoo, etc.: Google continue to be the most agile big company in the market, maybe in any market. The Twitter move proves once again how fast it can act (assuming it does act), and makes you wonder how anyone can compete with it. Google will integrate search into Twitter. Microsoft isn't left with much outside of buying Yahoo and continuing to establish itself as a viable number two.
Continued growth of the Twittersphere: Monetizing Twitter, or developing a business model for it, is not as much of an issue today as it was last week. With Twitter free to grow as a Google property, expect ancillary businesses such as Twhirl and Twittelyzer to pop up more consistently. Expect search optimization for the combined Google and Twitter to become a more popular discipline.
One other thing worth considering. When a privately-held hot internet company is on the table, Google takes it off. That's one of the reasons it has held one of the world's nastiest competitors (Microsoft) at bay. What's still on the table? Facebook, but Google won't go for it now. Does Rupert Murdoch now go for Facebook? That would add a new content-search play for marketers to consider.