Here's a "souvenir" Halladay ticket I bought after the fact...
(Barcode digitally removed from image.)
This is a "Ticketmaster" style ticket. "Ticketmaster" and "box office" tickets have the date of printing in the lower left corner, and as murphusa correctly points out in this case "A10JUN0," shows that this ticket was printed after the date of the event.
The Mets will reprint the Santana no-hitter tickets on "season ticket" stock. This is the season ticket for that game...
(Seat number, account number and barcode digitally removed from image.)
As you can see, there's no place where the date of printing is indicated. So, barring some deliberate subtle printing variation, those of us who have the season tickets - and especially those who got caught up in the heat of the moment and paid as much as $500 for one - are, at least from a collecting standpoint, screwed.