Sprint, and formerly Nextel, have been sponsoring NASCAR for over a decade. I dont know how long their contract is with NASCAR but Im willing to bet they make a big pile of money being the sole primary sponsor on every single event. As CanDman mentioned, there are probably a lot of people in the stands that hold a smart phone to get up to the second updates on their favorite drivers.
The point of making an announcement regarding a smart phone at a NASCAR event is moot. Its the fact that they are making an announcement about their future flagship phone which is important.
This argument has already gotten off topic enough so I wont bother with any further responses. To me, it seems like a logical business move. Where else would you announce your new flagship phone if it werent at the largest NASCAR race, where you were the primary sponsor, of the year?!