There was a thread a while ago, but I can't seem to find it either.
Anyway
1st - Country (J is for Japan, 1 is America, 2 is Canada, W is Germany)
2nd - Manufacturer (Toyota is T (except if your car was built at Toyota's NUMMI plant, then it's a N))
3rd - Model of car
4th - For 4th gen's "B" isfor 4WD models and "Z" is for 2WD models.
5th - For 4th-gen's, "T" is for V8 models and a "U" for V6 models.
6th - Not sure
7th - For 4th-gens it's a "7" for Limited models and a "4" for SR5 and Sport Edition models.
8th - Not sure
9th - I'm still trying to figure this one out, some have X's and some have numbers
10th - Year, "3" for 2003, "4" for 2004
11th - I think this is for plant. "0" is for Tahara, and "8" is for Hino
Rest of the numbers - identification number.
Someone correct me if this is wrong.