I'm unclear why they would buy it.
Twitter is not a heavy technology thing, Google could easily roll their own very quickly.
The userbase also could quite quickly be built up from googles existing users.
The brand obviously has value, but I'm skeptical google would offer what Twitter would be wanting.
I disagree. The reason twitter works is because that's all it is - a simple status update. Rather than a feature to set your status on an existing product. So google don't have it.
Google's wonderful at competing when they can change the experience paradigm (Gmail, Google Maps). They're terrible when they can't (Google Answers, Google Video). If Google tried to ape the look and feel of Twitter ("but better!"), they'd fail.
The brand obviously has value, but I'm skeptical google would offer what Twitter would be wanting.