No, ATP does not have a partnership with any airline.
First off, coming out of the ACP program you're not near experienced enough to work for an airline, and ATP doesn't pretend otherwise.
Second off, if you work for ATP as an instructor you reap up more twin engine time than most anywhere else (Unless you happen to work in Las Vegas in July, and people are too smart to attend flight school in 110 degrees weather. Like I do.) and airlines rather like that. /ubbthreads/images/icons/smile.gif
I've heard rumors that pre-9/11 ATP had an agreement with two regionals that they wouldn't hire more than X ammount of ATP instructors per month, since they were depleting ATP's instructor pool. Of course, that was pre 9/11..