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Google last week teased us all with a test of number porting. Today it's gone live for everyone. You're going to need to think about this a bit before you dive in, however. If you port your current cell phone number to Google Voice, your current plan will be canceled, and you may be subject to an early termination penalty if you're still under contract. You'll then need to get a new plan, with a new number, which you'll then instruct Google Voice to ring.
For some, that's a bit much. For others, a godsend. [Google Voice Blog]