I don't pay any carrier anything for SMS service. I use Google Voice, generally via my gmail inbox, for all SMS>
Then my iPad is running a soft phone, configured with Gizmo5 SIP credentials. So at home, calls to my Google Voice number ring both my iPad and iPhone. I'm using the jailbreak `backgrounder` app now, but iOS4 will offer a sanctioned solution.
This is my mashup for shitty AT&T cell service, fast cable modem, no land-line, and nice and shiny but power efficient Apple hardware.