Most sites I know of that use geolocation cause the browser to show a dialog asking the user permission to gather location info. This is like it can be seen here:
http://html5demos.com/geo and the way it works with Google Maps.
But Twitter seems to be able to do it another way. From twitter.com, when you compose a Tweet, you can click a little button, and it will add your location to your Tweet.
But in the case of the twitter implementation, there is no user interruption.
How is this done?