Many a customer of the top VoIP companies here in the USA have always expressed frustration when it comes to Local Number Portability. While it has existed for mobile phone numbers, it really hasn't been an "official" part of the VoIP world's regulatory environment.
What does this mean? Well for starters in my view this should not apply to "free" services that assign you a number to use such as Jangl or GrandCentral. On the other hand, if you are paying or paid for a number to receive inbound calls, such as Skype In, GizmoProject, Yahoo Messenger with Voice, AIM Phone Line or Voice 1.5 carriers like Vonage, AT&T CallVantage, Packet8, Earthlink's Truvoice, Broadvoice, then the number is yours to move.