My guess is, that the rust problem on the surface is in the states where you have snow.
But my wifes Touareg is proof that the salt is not the sole problem: Her Touareg is a 5 years old with 3 times the miles and drove the same days as I drove the Tundra and guess what: NO rust!!
Since the Tundra's are only a year or less old, I don't think we reached to top of the rust problems.
Just look here:
95-00 tacoma rust buyback program - Toyota Forums :: Toyota Nation
Chances are, that this is not a Tacoma problem alone and the frame rust problem is a problem you find even in the "non rust" states!!