Multiple Users and Multiple Inputs on One Machine?

BozoForPresident asks: "Not long after seeing a dual monitor setup for the first time I thought how useful it'd be to plug in another keyboard and mouse for a second user. That $4000 dual headed laptop (reported on Slashdot on Sunday March 16) becomes a more viable purchase when you add a couple of USB keyboards and mice for an additional user. Microsoft will never do it but how difficult would it be to make Linux handle 2 (or more) streams of input and direct them to their respective windows?"