sounds very cool. im in the same boat as you working all day and coding bits and pieces at night. i also had the same idea for trying to implement gps in my flex app. of course i am doing most of my development in OSX so there is no gps alternative. it seems the only way to communicate to any connected preps is through an external call to a java app or something like that. i havent put any more thought or effort into obdII just because i don't have any connectors to my car for that purpose. sounds like your app is coming along great though. i would be interested in trying out if you have a compiled version that you would be willing to release. you can download mine and try it out
here if you like it let me know what you think. maybe we could collaborate and make one really solid app. who knows. good luck with your development though.