GrandCentral, acquired recently by Google, is a new service that allows you to get one number that is then forwarded to whatever phone(s) you are currently using at the time.

I could see this being as popular as the universal email addresses (Yahoo, Hotmail, gmail) that allow you to mask your ISP and move around without changing your email address. Combined with voice-to-text messaging and Google's other services this could be pretty powerful for those of us that are not tied down to one phone.

They are currently in private beta (which you might be able to get in on through InviteShare), but you can sign up to reserve a number when the service becomes more widely available.