What Eric said. If you want to take an extra precaution any Lowepro, Think Tank lens case will do and then wrap it in a fleece sweater. Good way to carry an extra layer of clothing with you should the need arise.
Thanks guys. I wondered about your experiences. A couple years ago I carried (onto all the airplanes) my EF 70-200 in my backpack, in the soft sack with a fleece wrapped around it. When I got the to lodge in Alaska, the lens cap had come off and the UV filter was smashed. It made me nervous about humping the lens around.
Michael,
I humped my borrowed lens around it the Bob Marshall Backcountry on horseback for a week wrapped in a pair of heavy boot socks w/o so much as a scratch. Add the ziploc bag as suggested above and you are water resistant and shock resistant as well.
JW