The service sounds cool: You're assigned a uninumber and whenever anyone calls it, all your phones ring simultaneously. All your messages land in a central voicemail box.
So why would Google want to buy such a service? An offensive strategy assessment suggests that GrandCentral could be a "wrapper" service for all Google's carrier partnerships. Google provides phone apps and YouTube vids and helps carriers sell data plans. Google makes money from mobile AdWords but doesn't have to worry about hardware costs. As phones proliferate and GrandCentral adoption increases -- with Google's aegis, how could it not -- Google also advertises through the app. Plus, and more importantly, Google gets even more data about how consumers are using their phones, and uses the data to target ads more efficiently.
Here's another thing: GrandCentral also creates MP3s of all your voice messages. I wonder if Googles interested in mining those messages for advertising? That's the creepiest idea ever.