the back of the Altima's gauge cluster (the odometer in particular) is driven by a small motor with a worm gear on it. Over time, the worm gear becomes separated from the driveshaft of the electric motor, and doesn't register the odometer.
Take the gauge cluster out, remove the motor, and push the worm gear back on it. A little dab of superglue will fix the problem.
I had this same thing happen to a friend's 240sx.
There are all kinds of nissan forums available, but the best is Fresh Alloy
Hope this helps!