As an add-on, you MIGHT get away with it. As a stand-alone service, you'll be on your backside. I am guessing you've not seen the state of peoples beds? They just don't care what its like - out of sight, out of mind.
I am not sure steam cleaning is the way to go either, because of the drying times. Ewbank have bought out a small vacuum cleaner for matresses, with a UV light attached to kill the bugs. Even so, it is the dustmite feaces which do the damage, not so much the mites themselves, so its vaccum, vacuum, and vacuum again.
The other thing is, is that those who are aware of the fact that mattresses need vacuuming are the sort who do it themselves or ask their cleaners to do it (i often vacuum mattresses for people when i'm doing bed-moves).