Mobile Maps with Leaflet
Unlike other open source platforms such as OpenLayers and Google Maps, Leaflet was designed from the beginning with mobile phones and browsers in mind.
Leaflet's walkthrough at http://leaflet.cloudmade.com/examples/mobile.html shows how you can create a full-screen map and request the user's location. Note that people will have the option *not* to give their location, or they may wish to enter an address.