Several browsers (FF 3.5 and MobileSafari) support the new W3C browser stanard to do geolocation (http://dev.w3.org/geo/api/spec-source.html). This will work on desktop browsers as well as mobile browsers to determine a user's location.
The idea here is for a simple page that determines your location and provides 1-click access to local services (weather, human readable location, yelp, ATMs, starbucks, etc.) either as a link or a mashup using an API.
http://www.slideshare.net/rsarver/w3c-geolocation-api-making-websites-locationaware
Should optimize for iphone-like sites:
http://www.engageinteractive.co.uk/blog/2008/06/19/tutorial-building-a-website-for-the-iphone/