Here is an answer from another forum:
Ming Wu , Apr 08, 2008; 02:43 p.m.
I've brought 40D, 70-200 and 17-40 into both Dodgers Stadium and Angels Stadium. The security at the door are more concerned about outside food and drinks than the camera equipment. Every stadium seem to have a different restriction on bag size, so be sure you find out about that. I've seen people turned away at Dodgers Stadium for bags that is too big.
As far as inside the stadium goes, never had any trouble. As long as you are not blocking any aisle and isn't trying to get a "free" seat upgrade, the ushers pretty much leave you alone.
The real hard part is juggling the food, drinks and camera while trying to enjoy the game.