Too bad it isn't real time and able to find tennis courts anywhere that Google Maps has coverage. I guess, however, that all that image recognition is highly CPU intensive and thus he has to generate results for an area, cache them, and then deliver them.
The biggest problem is that I don't think Google would allow me to do that - from what I understood from their Maps TOS, the imagery is strictly for displaying to the visitor.
I'm sure that if you sent them an email telling them what you were doing they would allow it. (Either that or they would say "Hey! Good Idea!" and a few months later they would come out with Google Sports Courts, able to find basketball courts, tennis courts, football and soccer fields, etc.)
