1. The real play here is for identity and reputation management. That's where the conquer-the-world ROI Is.Update 3/26/07: I signed up with GrandCentral.com after all. Why? Well, in part to preserve a username that I wanted. However, I also figured out why this useful for me now.
2. Others will do this.
3. Your number lasts only as long as the startup
4. The switching costs are very high. So if you commit and the price goes to $400/year don't cry. Let me repeat: you are signing a contract in blood.
5. It will be buggy and unreliable. That won't fly well.
6. Various vendors have sold this sort of thing for years, but it's always been very expensive.
7. I'd wait for Google.
Nobody wants to carry two cellphones, but if you get a corporate phone you either need to carry two phones or be tied to a phone number that you lose when you switch jobs. GrandCentral solves this problem. I can get the corporate phone, but publish a number I own that redirects to the corporate phone. The number stays with me, even if the phone doesn't.