Just to add to that, the point of the 6-month wait is to prove that your animal didn't end up catching rabies the day before you vaccinated him. (Which is unlikely, but possible.)
So if he got bitten by a rabid animal on Dec 31, you vaccinated on Jan 1, he passed his bloodwork and those results came back Feb 1, by May 31 he could still be rabid because he was vaccinated after he was bitten. (The incubation period, which is how long it takes between getting bitten and testing positive/showing symptoms of the disease, is a few months.)
Obviously this is a highly unlikely scenario, but that's why the PETS scheme has the 6-month wait. (And why the bloodwork is done first, followed by the 6-month wait.)
Hope that helps clarify things. <End geekery>