If we can get live traffic info from car transponders (sort of like Google Maps does), we could possibly get live parking info too. Because we don't have always-on, internet-enabled car transponders, though, you'd have to get the data some other way. If Google Maps can get live traffic data from users, perhaps you could compare the intended destinations of those users to see if there is likely to be a parking problem in a given area when they all arrive.
Mokalus of Borg
PS - There would be a fair amount of guesswork involved.
PPS - But sometimes that's enough.