Ok so I searched these forums under various search results of "Power Seat", "Driver seat motor", "seat motor failite" etc, and found some different responses.
This seems like a common problem, as I found pretty much the same story every time. This story came out with a positive ending for one guy:
It appears there actually are TWO SEPARATE motors on the drivers seat, one for the back/forward motion and one for the up/down motion. Similar to what AceCombat mentioned, if you can expose the metal under the cover, try cleaning the contact points. In the post linked above, one guy got it working by having a friend push the seat forward while he cleaned/adjusted the seat. Eventually it worked.
So it might be a short in the motor, or the motor may be gone altogether, but you should be able to test if the motor is dead or not. If its not dead, then possibly a little wiggle and play, and some cleaner sold at radioshak and get to work.
Also, though not entirely Nissan related, but a quick search in google turned up many more similar stories. One in particular mentioned a fuse that controls the driver's seat. I don't know if this applies to Nissan vehicles, but take a look into it.