Is Arduino a good platform for monitoring 24VAC thermostat circuits?
It's perfect.  I did this exact thing in my house.  I used an MID400 optocoupler to turn the 24VAC present across the terminals of the thermostat (at the furnace end) when the thermostat is NOT calling for heat into a digital high at the Arduino.  I'm using an XBee network, but Ethernet (or even a tethered computer) would work just fine, too.  It'll work for higher voltages, too.