Source: http://feeds.gawker.com/~r/lifehacker/full/~3/1dcx6KTYRv4/google-voice-to-allow-number-porting
We've been very excited about Google Voice ever since we gave it a closer look, but our biggest cause for hesitation (and the biggest complaint of many of our readers) is that you have to change your main number to take full advantage of Google Voice—a process that's always a pain for both you and your contacts. If you feel the same way, we've got good news: Reader Reid points out a Google support page explaining that number porting may be offered in the near future.
UPDATE: Reid writes back to point out that Google already allows you to take your Google Voice number with you to another provider if you decided to leave Google Voice at some point down the line.