Android Desk Phone Tablet
For external headsets you would be better looking at the incredible range of bluetooth enabled headsets for mobiles. I doubt that there will be any support for the current desktop usb based ones anytime soon as its not the focus of Android atm. A hardware vendor would have to write the drivers for them, or an opensource effort. 

I was just playing with the new native sip voip on Android 4 and its now deeply integrated. I was using the voip account that my home phone uses so my when someone call my house my mobile also rings and uses the wifi, very cool I wonder if this will pick off.