I looked into this awhile back and found two things that may be of interest. One is that there are thermostats available that can be controlled via a phone interface. I think I also saw some that could be controlled via computer too. I think the phone control idea may be more practical, because that way you can change the temp by calling in from anywhere, say on the way home from the airport.
The other device I found was a telephone dial in controller that had a couple of outputs from a relay that you could use to control anything. Using that, you could use the phone to switch between two thermostats. You could have the main thermostat set to hold at say 45, and leave the aux thermostat always set to 70. Then, on the way back home, you could call in and have it switch.
It looked like they did a pretty good job on this unit, including ability to work on same line as an answering machine. You can set the unit to pick up on say ring 5, while the answer machine picks up on say ring 3. To accesss it, you call in once, let it ring a couple times, then hang up. When you call back again within 30 secs, the unit will answer on the first ring. They have access code protection too, of course.