If you purchase an Android phone or tablet with AT&T between now and July 17th, then you'll get $50 worth of Google Play Store credit.
This doesn't apply to any phones you'll buy without a plan; you'll need to purchase your phone with an installment plan or two-year contract agreement. If it's a tablet, you'll need to sign on the dotted line for two years.
The $50 voucher will last you until May 15th, 2015, so if you don't spend it all by that time, then you'll be out of luck. One other bit of information: the $50 credit cannot be applied to equipment purchases throughout the Google Play Store. So if you were thinking about getting a nice new phone, tablet, or chromebook, then think again.