The A/C button bulb is definitely replaceable...I've done it!
Once you slide out the A/C button assembly (the long rectangular piece with the button) you need to carefully separate the housing. There are four little tabs that hold the two halves together. You need about four hands to try and get them all at once, without re-engaging, but it is possible with a few small screwdrivers. Once you separate the halves (be careful, as I recall there is a spring in there that wants to go flying) you can access the bulb. I replaced mine with the 12V Radio Shack standard bulb. It is in a similar housing with the twist base like any of the other small dash bulbs.
The RS bulb was a mm or two longer, which caused it to catch on a small nub of plastic once the two halves were snapped back together. The button would still function, but it would get hung up and not fully push back out because of the friction. I took the housing apart again and ground down the nub (three seconds with my Dremel) a few mm and that was enough to gain clearance for the bulb. Reassembled and it works good as new.