There is something about the FAA ATPL that hasn't been mentioned here:
You NEED to have 1500 hrs in your logbook BEFORE you can take the written (computer) test!!!
And that is the difference between JAA and FAA. JAA accepts you to do the ATPL written test any time after you got your CPL. But as long as you don't have the 1500 hrs it is called "frozen" (sonds better than "useless", doesn't it?). And many airlines have come to the questionable custom to require this "frozen" thing to hire you even when only a CPL would be necessary (and only that's what you actually have at that point).
In the US you have to present your logbook to an FAA inspector in order to get the permission to take the ATP written test (don't know exactly if a DPE can also do this...).