Kevin, what your heating guy said is a very common practice, but is not considered to meet the electric code.
In practical terms, with household HVAC, you can get away with this violation; that is, it'll work. That doesn't make it right, though.
NEC specifies that when wires of different voltages are run together, the insulation of all wires will meet the rating of the highest voltage present. Since low voltage wire is usually tested/rated for at least 100 volts, and household current isn't much higher than that, there usually isn't a problem- but it doesn't hurt to make sure that the t-stat wire used has (is marked) for a higher voltage.
NEC also specifies that control circuits will be kept in separate raceways than power circuits. This is because current running through a 'power' conductor can induce a voltage in a 'control' conductor, which will fool some types of controls into operating. Once again, since household HVAC usually doesn't use these sort of controls, you can get away with "cheating."
Indeed, running the t-stat wire together with the power wire, at least for the short run from the disconnect to the airconditioner, is probably the most common code violation out there.
As for the NEC on disk, well, the best place is this site. If nothing else, you'll get the latest version- the earlier CD for the 2005 NEC had some glitches.....or get it direct from the NFPA (not reccomended except as a last resort....and I'm a member!)
