Oh, had I known so many things before I got my Nest I would have dumped states for every possible thermostat change on my old Venstar... The Nest plugin (which I'm forking to tweak because of this kind of issue) doesn't really update any of the standard HVAC states, such as when the fan is running or if the heat is on versus if cooling is on except through proprietary states (something I intend to remedy with my fork).
So my question is this: if the furnace comes on do normal stock thermostats update the fanIsOn attribute to true and then toggle to false when it shuts off? I would assume so but I'm writing something that depends on knowing when the fan is running and have no way to currently test it other than the play devices - which don't work anyway because apparently the sample Insteon devices don't honor the "- sample device -" method like X10's do and there is no X10 thermostat (at least not in my database they don't).