It just warns you and says that "same application featurews may not be supported" or something like that. TouchDevelop and some of the Compass apps do this.
I was wondering though if there are any apps (or if developers have the option to set this) where an optional device feature such as the gyro is so integral to the app that it won't even install without the device capability. This is what I haven't seen yet - though maybe the marketplace is already filtering them out for me.