The 17-liter Gregory Maya ($99, gregorypacks.com) might be the best bet for you. Brand new for spring 2009, Gregory designed this pack with "bio-synch" suspension, meaning that it has two attachment points, one at the shoulder and one at the waist with a belt designed to ride up much higher on the torso, which allows the pack to move with your body instead of forcing your body to move with the pack. The Maya also has a fleece-lined pocket at the top for important stuff, a discreet pocket for personal items, and plenty of space for an emergency bag of treats (but I'd avoid melty chocolate).
If the Maya sounds too small, step it up to the 35-liter Jade ($149), a more traditional women's frame pack with dual hydration ports, an auto-fit harness system, nice venting suspension in the back, and an all-access front zip. Try this one in a small.
Or go minimalist with CamelBaks three-liter L.U.X.E. ($90,camelbak.com). Designed for mountain biking, the L.U.X.E has an amazing amount of overflow storage space for extra layers, an emergency first aid kit, a few energy bars, and 100 ounces of hydration. The narrower women's specific harness curves in all the right places and the shorter footprint allows for a shorter torso. Bonus: CamelBaks come with a bladder.