So if I port my real cell number over to Google Voice and my cell plan gets canceled, then isn't Google Voice basically useless because I wouldn't have a cell to forward it to? It'd be nice if they could work with carriers to assign you a new number and transfer your existing to GV in one transaction.
The idea is that you then get new service, with a new number you can use as your primary number with the old one still forwarding. Basically the equivalent of a forwarding address.
However, this may be preparation for the other shoe to drop. The other shoe would be some kind of data-only plan for Android phones where your actual phone service is simply VoIP with your Google Voice number, over the data plan.