I've brought my Canon DSLR with the 70–300 mm lens many times. It's a DO lens, so it's smaller than the average long-distance lens when I have it zoomed all the way out, and I make a point of turning on the zoom lock when I'm not using it (mitigates the risk of the lens extending when I enter the ballpark). What they're really concerned about when it comes to cameras is making sure that you won't obstruct other fans' view of the game. Their main goal is to prevent people from bringing in those big white lenses that the press photographers use.
For Opening Night back in 2008 I brought two lenses (the aforementioned 70–300 and my normal 17–85) and had no issues. Of course, security that night was run by the Secret Service and so normal rules might not apply.
The ONLY place that has ever hassled me about that 70–300 lens was an usher at the FU Center in Philadelphia for a Caps–Flyers game last season, and I suspect part of the issue there was that I was using the lens hood and it made it look a lot bigger than it is. Of course the Flyers' and their arena's policies say absolutely nothing about cameras, but they insisted I'd need a press pass to take pictures. At that point I said "to hell with it" and put the camera away since the Caps were losing 7–1 anyway. Only other time I came close to having an issue was when I went to enter RFK for the Nationals' first home opener back in 2005. I didn't have the 70–300 lens then, so I brought my 17–85, and one of the RFK staffers tried to tell me, before I reached security, that it wasn't allowed because they didn't allow "professional cameras with detachable lenses" (that is indeed what RFK's rules stated). I told her, without breaking stride, that the Secret Service could send me back to the car if it wasn't allowed. Of course there was no problem.
The only really strict camera policy I know of is at Hershey Bears (AHL) games, where anything bigger than a small point-and-shoot isn't allowed. They have a sizing gauge taped to each door and if your camera's body is bigger than about the size of a credit card you have to take it back to the car.