i got this dakine apex bag for xmas. it hold the helmet on the back of the bag instead of the bottom or inside, so you can still carry whatever else you need. its got tons of cool little pockets and stuff for whatever little things you need to carry and is big enough to fit my 15" mac book pro. it also comes with a separate pocket and 100oz camel back style reservoir. i haven't used it much yet, but it seems like a great bag.
http://www.dakine.com/bike/bike-packs/apex/
