So the problem is you can't necessarily send or receive an SMS without a device on a cell network. I suppose you could add Bluetooth connected support, or sync across the Internet in some way with your device. So if your phone is either local (Bluetooth) or on and connected to the Internet, then via your computer you could send/receive SMS/MMS via syncing.

Not sure useful to me, but it's not as simple as just creating a desktop application.

THIS IS AN AWESOME IDEA!

Or an iMac desktop version?

THIS IS A GREAT IDEA.