Rinkrat, try using these arguments:
Right before X'mas, I went to the Disney on Ice show at Staples Center in LA where Lakers and Clippers play. I rented the 70-200 2.8 IS lens, but I knew they wouldn't let me take it in, so I put the lens at the bottom of the backpack and put baby diaper, juice bottles and other stuff (we took our 2 & 4 y/o). I put my 28-135 IS lens on my 20D. They didn't see the 70-200 in the backpack, but they wouldn't let me take the 28-135 lens because it's about 1/4 inch longer than the long side of the guy's ID card (same size as a credit card). We went back and forth for like over 5 minutes, then his supervisor came and told the guy to let me in.