In case you haven't seen it, From Dust uses fluid simulation as a game mechanic. It's a god game, with flowing water that erodes rivers etc, at a constant 30 fps. There's an xbox demo, and a PC demo to come. Here's a video: http://www.youtube.com/watch?v=gSOQGazo7Oo&t=33s (I think the fluid grid is slightly coaster in the actual game, but water basically flows the same). BTW: I think it's fantastic as a toy, less good as a game.

However, from what you're saying, the fluid simulation isn't as expensive as I thought.