Windows has had a "Use this computer as a Bluetooth Headset" option buried in the Bluetooth preferences for a long time now.
The Address Book in Mac OS 10.0 and 10.1 would also let you send and receive SMSes with compatible phones (no joke):
http://www.oreillynet.com/pub/a/wireless/2002/11/27/sms.html
And Linux has had a few projects (like nohands) which do the same thing.