Curiosity about hardware buttons on 4.X

[BlackJath]

So HTC uses hardware buttons on it's new ICS phones, so long as it works I suppose it doesn't really change anything. Except, HTC knows this and adds into Sense 4 that if a menu button is needed to add it to the bottom of the screen. I have a feeling that isn't quite the normal way AOSP works, so will other ROMs, like Cyanogen, have a menu button that shows up if it's needed unless specifically modified for HTC's new phones?
Not a big deal I suppose since it's supposed to be falling out of use anyway, but I'm curious if old apps will have issues with that.