I just installed my system almost two weeks ago, and last week I bought a relay I'm going to install this weekend for use with the output. My keypad is mounted right next to my t-stat. My t-stat is an older Enerstat model (I think the same as what AprilAire is now). It's not programmable, but it has two temp settings (day/night). You can toggle with a button on the stat, or also with a contact closure. In the past, I used an X10 module to automate switching between the two settings. Now I'll do it with the M1, and the wire will already be run for me!
When my stat came out, it had the option (which I took) to be modded to allow future upgradeability to rs-485, but I never installed it. The need to get beyond two temps just wasn't great enough. I don't even know if I could still get that upgrade done. But I could now easily get multipe temps by combining some rules and the M1 keypads built in temp sensor... have the two settings in the t-stat be at the two extremes, and then if I want an intermediate setting, use rules to switch between the two extremes based on keypad temperature. B)