I got caught last year on this. AAC policy states no professional cameras or lenses over 4" in length if you don't have media credentials.
I didn't have too much of a problem with my 20D but my 70-300 (non-DO) is 5.6" and they wouldn't let me in.
Here is the link and actual text:
http://www.americanairlinescenter.co...age/PageID/128
Professional cameras are prohibited at all events in American Airlines Center unless accompanied by appropriate media credentials. Cameras withouta flash may be allowed for some events. Cameras with lenses that are 4 or more inches in length are NOT allowed. Video cameras and audio recorders are prohibited in American Airlines Center unless specifically approved. Please check in advance by calling 214-665-4299.
Restrictions on the use of still and video cameras vary by event. The event producer determines this policy. Some sporting events permit still cameras without flash, but not recording devices. Concerts typically do not permit any cameras or recording devices (audio or video). Guests should contact an American Airlines Center representative at 214-665-4299 to find out the policy prior to the event.