I don't know how far you want to go with this, but if you are looking to stay with X10 for the limited control that it gives, you can get the X10 thermostat setback controller. I cannot give any information on how well they work or not as I have never tried one. If you want to get a little more elaborate, you will need some type of comunicating thermostat. The one I have installed is an RCS-TR40 thermostat that I picked up on ebay for just under $60.00. The TR40 has been replaced with the TR60, but if you can find a TR40, yo may be able to get it at a reasonable price. When I bought mine back in October, the TR40s were going for a typical buy it now on ebay for $175.00, so the price I paid was a steal. It runs on an RS485 serial line which I have a cat5 line to my HA PC. I know they also have ones that run on Z-Wave and a few other protocols. RCS (Residential Control Systems) is a good manufacturer for a few different kinds of thermostats.
My software RemoteWatch X10 has the capability of monitoring an RCS TR40, and displaying the temp and all the LED indicators on the unit. It may even work for the TR60 as I am sure the protocols are similar if not the same just with other added features. I do not as of yet have the ability to control the thermostat through the software yet, but that is a feature that I have on the list of TO-DOs for the software.
Hope some of my babble helps.
Dan Bemowski