The caveat is that you cannot make calls to other home or mobile numbers, nor Skype for now. If you have broadband (duh) and need a phone line to authenticate your TiVo or satellite box, activate credit cards or give your parents a way to call you that doesn't use up your cell phone minutes, then this is the story for you!
Got broadband? Add phone service for $0/month: http://makezine.com/11/diyhome_voip/
Oh, if you get your friends on Free World Dialup and THEN you can call each other and get off of Ma Bell's back!