I'm analyzing tweets and need to find which state (in the USA) the user was in from their GPS coordinates.  I will not have an internet connection available so I can't use an online service such as the Google Maps API to reverse geocode.  
Does anyone have any suggestions? I am writing the script in python so if anyone knows of a python library that I can use that would be great.  Or if anyone can point me to a research paper or efficient algorithm I can implement to accomplish this that would also be very helpful.  I have found some data that represents the state boundaries in GPS coordinates but I can't think of an efficient way to determine which state the user's coordinates are in.
Use a point-in-polygon algorithm to determine if the coordinate is inside of a state (represented by a polygon with GAP coordinates as points). Practically speaking, it doesn't seem like you would be able to improve much upon simply checking each state one at a time, though some optimizations can be made if it's too slow.
However, parts of Alaska are on both sides of the 180th meridian which cases problems. One solution would be to offset the coordinates a bit by adding 30 degrees modulus 180 to the longitude for each GPS coordinate (user coordinates and state coordinates). This has the effect of moving the 180th meridian about 30 degrees west and should be enough to ensure that the entire US on one side of the 180th meridian.
Determine the US state from GPS coordinates without using online service