If you're not familiar with moola, check it out at www.moola.com
. I want to see a similar site, but using bitcoins instead of $$$. Someone make it.
EDIT: The basis of the Moola site is that it gives out a free penny if you have a zero dollar balance. The same premise should apply to a bitcoin site as well - give the player a free bitcent (or probably less, given how much more a bitcent is worth than a USD), then let them play against other people to try and move up the ladder. Pay for the free BTC by showing the players ads (either video ads, or just general ads on the site).
I also particularly enjoy the gold rush game, so something like it would be a requirement.