Recently I am thinking about creating a small 3Dmap in Unity3D, and I have some buidings which were created in Blender imported into Unity. I am going to add a search function to my unity map which has the similar function with the google map, the difference is that my map is for buildings not the cities. I am new to unity, forgive me if my qeustions are meaningless to you. :D
Assuming you have a lat/lon mapping into Unity space, you can use the Google Geocoder. It takes a text string and gives you a lat/lon coordinate.
Official geocoder docs: http://code.google.com/apis/maps/documentation/geocoding/
Here is a interesting looking geocoder library, haven't tried it, but it seems nice. http://code.google.com/p/geocoding-net/