First off, this isn't quite the same as transferring a phone number from one carrier to another; your number still works with your cell phone or landline. Instead, Google Voice will intercept calls going to voicemail, thereby providing you with transcription, e-mail integration, and the other goodies that Voice users have come to rely upon. Secondly, if you want to use call screening, forwarding to any phone, or other features of the service, you'll still need to give out your new Google Voice-assigned number.
In either case, all Google Voice users can place outgoing calls, free to the US and Canada, and cheap elsewhere (but you'll still burn cell minutes if you use your handset).
You'll need an invite to get access to Google Voice, but invites have gone out to many existing subscribers, so ask your friends. If you want to use Google Voice voicemail, check Google's supported carriers to make sure that yours is included (all major providers are).
Google has published a help page with details on how this will work after you activate the feature. Sadly, iPhone users won't be able to use the handset's Visual Voicemail feature if they activate this service, but apparently, you can turn the feature on and off on the fly, in case you find you prefer your phone's integrated voice messaging.
