There is a part in the ATP requirements in the FARs that reads something to the effect of "a SIC performing the duties of PIC while under the supervision of a PIC" can count those hours toward obtaining the ATP. However, that doesn't apply to most 121 operators, since most of our aircraft require a type rating. Since you don't have the type rating, you are not qualified to count PIC time. So a lot of FOs are going to feel that pain come upgrade time. One of the drawbacks of jumping into the airline game without the hours first. Luckily, I have the hours, because I sure can't afford to rent a plane these days.