At a media event on Wednesday, Chad Evans, senior vice president of mobile product development for MLB.com, explained that Passbook tickets are first scanned, first served: Once a Passbook barcode is scanned, it is no longer a valid ticket for someone else to use. So, think twice before you share a screen shot of a Passbook ticket to show off this new feature. On the bright side, this does mean that you can give a ticket to someone else if you can no longer make the game--just forward on the e-mail confirmation to someone that also has Passbook, and he or she can open the ticket from there.

If you buy multiple tickets to one game, for, say, your family or friends that are joining you, you can either store all of the tickets in your Passbook app, or you can forward ticket confirmations individually.

Why introduce Passbook to just four stadiums? Its availability has to do with scanning technology, according to Adam Ritter, senior vice president of wireless for MLB.com. Every employee at each of these four parks has been trained on how to handle Passbook tickets, and these parks have the proper technology. MLB Advanced Media expects to integrate Passbook with more parks in when the 2013 season gets underway next spring.

Apple has big plans for Passbook, touting the feature as a way not only to store electronic tickets to sporting events like baseball games, but also boarding passes for flights, coupons for stores, and customer loyalty cards for businesses. Passbook works with iPhone and iPod touch models running iOS 6.