Hot Air Balloon pilot reports entering IMC inadvertently shortly after takeoff and using GPS and altimeter to navigate to a safe let down point.

Narrative

Morning flight. Surface winds (first 300 FT or so) flowed east to west; while winds aloft moved west to east. Preparing for flight required laying out my balloon with the surface winds in mind; east to west. Prior to inflating my balloon a visual check of surrounding conditions was made. No problems were identified. Once inflated; preflight safety check complete; I began flight which; due to the surface wind direction; was toward the west. At (approximately) 350 FT AGL the balloon entered the west to east wind flow. As it did I turned from facing west to facing east to begin piloting in that direction. It is at that time I saw the fog; which was now rolling in; VMC to fog dominating surface through 600-800 FT took place very quickly. Inability to identify surface conditions/obstacles forced aborting 3 attempts to land immediately after takeoff. Rather than continue to use fuel attempting to land where no surface detail could be identified; I flew on to an area where surface conditions were known to me; GPS coordinates were used to navigate to this location. On arrival; horns blaring and people clapping could be heard. Occasional breaks in the fog provided enough visuals of the surface to make a controlled landing; which was made. No injuries. No property damage. No damage to the aircraft. Altimeter; and GPS device with coordinates for a location I had a reasonable knowledge of were critically important to the safe conclusion of this flight.