I use a camelback resevoir inside of an Arc'Teryx 'Quiver' pack (http://www.arcteryx.com/product.aspx?prod=1352
). It's all perfectly light, not noticable while skiing. The pack flips around to your front side when you get on the lift or when you need to pull something out of the pockets. It's big enough for the camel back, snax, sunglasses, hat, glove liners, a not-to-bulky extra layer, and (when I ski alone) a book to read over lunch
That pack did, however, help me break a couple of ribs last year. I took what was probably my highest speed wipe out ever; caught an edge somewhere in the ensuing tumble and slamed
onto my chest - right on the buckle of the pack. And guess where my ribs cracked? Ouch.
A camel back by itself is definitely not noticable while skiing.
Camel backs are pretty damn tough. If you break one in a fall, getting wet will be the LEAST of your problems.
Depending on how close you keep the water to your body heat, you may also want to get the insulation for the tube and the cover for the mouthpiece. Helps keep the water in the tube from freezing.