I think that's a really cool idea and a golden opportunity to share our lives and experiences. I've often considered a web based service that connects us with like minded individuals who share the same openness to chance encounters, and want to help others explore the world, explore a city, a culture, or just their own neighborhood. Somewhere that you can find others who'll open their door, or simply ensure a few local tours and sound recommendations to intrepid travelers. I'm in LA, were I in SD I'd take you up on that in a heart beat.
Good luck. If I can help in any way, PM me.