I really, really doubt this is possible. There have been human populations that have been cut off from the rest of humanity for long periods of time, like the Australian native populations, and they didn't become a seperate species (as there is no group of humans that can't breed with others to create a fertile child). Considering that I can now fly to Australia in less than a day (and potentially get busy and breed), it just isn't going to happen in any time frame that is worth worrying about. We'll be gone as a species from something else long before this is possible.
Edit: Probably the biggest reason we don't see any sub-species in humans is that humans respond to differing enviromental pressures mainly by changing behaviors. Like how we didn't evolve fur when we moved into colder climates, we modified our clothing.