I'd agree with Dave and say the market is close to non-existent for iPhone apps for (real world) racing, though I will say that the racing games market could be a nice little earner.
It's very, very hard to make money anymore just developing mobile apps - it's like the movie business; an occasional hit, with many failures. The subscription model would help but you are entering a full time deal then I think.
I've seen players at the track use ADW apps for betting. I haven't really found any other scenario when players feel like they need mobility over a computing power (as your poll a few weeks ago showed).
I'm considering building a dedicated native app for
thorobase, but it already works well enough on the mobile Safari browser (save for those YouTube videos that aren't h.264) so it's a low priority.