Where to start... location awareness?

I'm working on a restaurant reviews app and I'm not sure what to use for location awareness.. I am developing it in ruby on rails. What I wan't it to do is have the ability to search for places nearby a user's 开发者_Python百科entered location or guess the user's location based off their ip address as well as off of geo coordinates or w/e too. Honestly I don't know where to start anyone have any good api / gem / article suggestions?

Check out the geokit gem for the backend, and the WC3 Geolocation API for getting location info from users.