I'm paying them about double their electricity costs, and they think it's a great deal. Being a greedy b*st*rd, I don't want them to discover bitcoins and start mining for themselves.
LOL... Dude, I don't think you really need to worry about it. They'd almost certainly rather have the cash, no need to sneak around.
Hell, even
as someone
already interested in BTC, I would take
1 ...
(crunches the numbers) ... USD$800/year/5870 to run a miner or ten for anyone interested. In fact, make it twenty, and I'll give you a discount for effectively paying for my home heating.

As for the ethics here - I really don't see a problem. You offered to more than fairly compensate people to run a program for you. If they don't insist on knowing what it does before agreeing, not your problem. Not like it does anything illegal, like act as a CP FTP site without them knowing or a spam bot or anything like that.
1) Serious offer, if you want it - Minimum six month commitment, payable in advance (since I currently own zero 5870 cards)