My D90 came with remote keyless entry. The previous owner had them installed at an Al&Eds shop. They work great. Mine only operates the two doors, and not the rear SW door.
If you really want to do the rear door too, you'll have to purchase a special rear door latch assembly, which is an option on the new european defenders. This unit has an extra "rod" comming out of it, where the solenoid attaches. They had one on ebay for $55 if I remember correctly a while back.
hope this helps.