Monica is not correct about only registered students being able to purchase tickets. Even though I did my undergrad at UT, I wasn't a registered student last year so that would seem to preclude me from her list. Not to mention there are people who are coming from completely different undergrad institutions who already have gotten the email (I've spoken to a number of them who were confused by the system).
Texas Box Office and the Athletics Office (Bellmont) are notorious for screwing things up when it comes to student season tickets, which is one reason I was apprehensive about the whole process. Something happened between the law school giving Bellmont the list of people who have turned in their letters of intent and this email going out. I don't know who's to blame, but I suggest taking matters into your own hands if you have yet to receive the email. You can call Bellmont (don't call Texas Box Office, they are useless) at (800) 982-BEVO or (512) 471-3333 and explain your situation. You may have to hold for awhile, but be persistent and you should get it sorted out.