What if you want to offer transit directions from your location to a rental?
Again, sorry if this sounds like I'm poo-poo'ing your effort; that's not my intention. I think there's another side to this story, though.
EDIT: Forgot to mention my own personal white whale. Google's geocoder is far from perfect, but it's the best I've seen. I worked for years with them, and none of the open source geocoders (or expensive ESRI products) could deal with addresses like Google's. Geocoding is like web WYSIWYG editors; they all suck, but some suck less than others. And everyone thinks they can do it better.
Actually, Google doesn't let you do that either. They have separate licensing agreements with every transit authority, so there is no API access.
You're right on the geocoder, though. The closest I ever found was Yahoo's, but they're still not up to much.
Lulz wht? Google's Geocoder is by far the suckiest of any of the ones I evaluated. I'd highly recommend Bing or Yahoo before Google's geocoder.
See http://3liz.fr/public/osmtransport/index.php?country=France&... and http://öpnvkarte.de/?zoom=12&lat=52.50944&lon=13.393... for examples of OpenStreetMap public transport data in action.