is a browser-based MMO that I've been running for a couple years. A friend told me about bitcoins a couple weeks ago and now (after a weekend of coding) they are accepted at Minethings.com. The game is free to try out and in-game purchases are optional.
I like the way the you work the bitcoin mining into your payment page.
I could spend alot of time in minethings.com if I'm not careful.
Just some ideas:
Maybe you could go even further and have bitcoin be one of the things you can mine in world?
Could you add a way to exchange credits back for bitcoins somehow?
Could you trade items for bitcoins and visa versa (between players) in game?
I think any of those would make your game uniquely fun and addictive.
Edit: I see now that your Terms and Conditions specifically prohibit trading any in game assets for external currency, so I see most of those are things you've considered and decided not to do. Still a very fun game. I'm up to 10 melds and thoroughly addicted. I really like that I can just visit it once a day. I'm too busy right now for much more than that.