What's in it for the game developer? Typically, they'd want to be able to control the in-game currency and have a monopoly on exchanging it for other currencies.
I'm sure there are many ways in which a game developer can benefit from that, I haven't put too much tought on the possibilities, but the first one that comes to my mind is a transaction fee of each Bitcoin transaction. That surely would be enough incentive to use Bitcoin as in-game currency, as it would remunarete devs for their work.
Now, someone with a much better understanding of economics than me, needs to look at this opportunity and think on how to go to accomplish that in a profitable way. If Bitcoins can be used as in-game currency, benefiting game developers, and game users themselves at the same time, that certaintaly would be a brilliant way of taking Bitcoins to a much safer level.