This is from Bill Livingston of the Plain Dealer, in his 9/11/12 column 'Weeden the man for better or worse' :
"The passer rating formula is hard to understand, but the general parameters of performance are well known. Perfect is 158.3. Anything in the mid-70s is mediocrity. Single digits in passer rating is the football equivalent of one of those relief pitcher outings in baseball when no batter makes an out, the hits keep happening, the outfielders are gassed and you can't print the ERA because no one has a sideways "8" on the keyboard to symbolize infinity."
FYI... ∞ is opt+5 on Mac keyboards.