alright, my opinion:
there will be several things we have to take a look at:
- gravity acceleration will be around 9.81 m/s², all the time
- there is some friction involved, as the wheel has contact with the treadmill, which will remain constant if the surfaces don't change
- there is no air friction involved if the wheel stays in place i think (air friction will, for example, keep stuff from falling at unlimited speeds, as it, at some point, counters the gravity => thats why galileo's experiment only fully works in a vacuum)
well, with all these constants, I'd think the treadmill will have to constantly gain speed as well to counter the (gravity - friction) value.