They moved it to 630 for the latest version, which is what, 2005?
The derating or as the NEC likes to put it "multipliers" are as follows:
Duty Cycle = Multiplier
100% = 1.0
90% = 0.95
80% = 0.89
70% = 0.84
60% = 0.78
50% = 0.71
40% = 0.63
30% = 0.55
20% or less = 0.45
I'll give credit for the effort of retyping this to another poster on another board.
One caveat is that dedicated welder receptacles or any dedicated circuit should technically be labeled as such. These are not general pupose circuits. I don't tho.