It's the eternal struggle: balancing the risk of skin cancer with the risk of terrorism:

The brilliant reasoning here is the same one behind the liquid ban on flights, but here's the kicker: security allows you to put on sunblock before throwing it away. So, if you apply the lotion on your skin, essentially confirming it's not liquid explosive, you still have to throw it away. Yankee Stadium sells a 1-oz. bottle of SPF 15 for $5. 

http://blogs.villagevoice.com/runninscared/archives/2008/07/bake_me_out_at.php