I was looking into that a year ago - I was planing on calling my computer (which was in the trunk) from the front seat so I could turn it on and off. OKAY OKAY....I KNOW, why not just wire the power switch upfront - It sounded like a cool idea.
Anyway, I ran into a few hardware mechanisms while searching google.com - They would plug into your phone line and then connect that to a serial port. From there it would tell your computer what to do based upon what you dialed (one for shutdown, two for reboot, etc). Thought it was a pretty cool thing except I needed 2 cell phones as well as $350 for the unit. They were mainly for large company servers that would crash and need to be rebooted.
Im sure there is some software out there, but even though it is fairly simplistic, i doubt its cheap. But search around for a bit. I think i was searching google with the phrase "computer remote shutdown restart" But that was a year ago and I wasnt looking for software.