A few weeks ago I shared some tips for leveraging the HTML5 geolocation api's. The API is great, and opens some very cool possibilities, especially when coupled with Google maps API's, but the one weakness is the browser implementation of user preferences when it comes to sharing your location with a website.
Maybe if it wasn't so inconsistent between IPhone and Android devices, it would be more managable, but right now it can be downright confusing.
Android's stock browser has a bit better implementation in my opinion. The first time you come to a site that requests your current location you get prompted on whether or not you want to share your location with that website. Whatever you answer, the stock browser will remember forever. You never get prompted again, it will just continue to share or deny access to your location for that website. If you do want to change the answer, you can go to Settings (from the Browser Menu), then Content Settings, then Website Settings and actually control your location settings for each website.
IPhone and Mobile Safari is a bit more complicated. The first time you come to a site that requests your current location you get prompted on whether or not you want to share your location with that website. But unlike Android, you do get prompted a second time if you refresh the page. As long as you answer the same, after the second prompt you will never get prompted again. Plus there is no way to change your location preferences for individual websites. The only way to change it, would be to nuke all your location settings by going outside of safari to settings, General, Reset, Reset Location and Privacy. And that removes everything. You actually get prompted to allow safari access to your location before getting prompted again to allow access to the website, bringing the total number of prompts to 3 if you change your mind.
In either case, if a user deny access to a website, talking them through reversing that decision is daunting at best, and you should give it some thought when you figure out when and how to prompt a user to share their location, because chances are, you will not get a second chance if they deny you.