in my experience (going back to ~1998), the best thai food is in thailand, though you can get comparably good thai food in cali (sapp and jitlada in LA!). when i visited thailand, most thai food was sold on the street and out of 'garages', with seating that straddled inside and the sidewalk. sit-down restaurants were relatively rare and mostly catered to westerners and rich asians (the food generally wasn't any better, but it was pricier). ingredients in thailand were super-fresh (especially the veges and fruit), so i can't see how that's a real advantage of american thai food.