Actually, Tacomas, Tundras, Sequoias, and even 2004+ Siennas have "5" as the first number.
My Tundra, built in Princeton, Indiana, starts off "5T" and so have all Sequoias I have seen. So do CA built Tacos, so I would be willing to venture a guess that "5" means a truck based vehicle, or a vehicle built on assembly lines that can accomodate trucks. The last part is meant for the 2004+ Sienna, which is also built at TMMI (moved from TMMK) and carries a "5" for the first VIN number. So "5" does not mean exclusively NUMMI-built.
My Avalon, built in Georgetown, Kentucky, begins with the number "4," as do Camrys that come from that plant.
To sum it up, "5"s come from plants that can build trucks (like TMMI), and "4"s from those that cannot (at least for now), and build mainly just cars (like TMMK). I think NUMMI separates Corollas and Tacomas by using a "1" and "5" repectively, with "5" being the truck and "1" being the car. Only TMMK uses "4" AFAIK. The rest are pretty much as Hachiroku said, and I would also add "W" for Germany (at least that's what it is on my sister's non-Mexican VW, an 84 Vanagon camper van).
My guess is that the next gen Tundra which is going to be built in both Indiana and a new plant in Texas are more than likely going to carry "5"s as well.