Mountain biking and hiking are not all THAT different, but I do both and see the need for a pack that can go both ways.
The Novara Commuter Pack
A pack I've grown very fond of this past winter, as a bike-commute pack, is REI's Novara Commuter Pack ($69.50). Don't let the name fool you. This is an excellent all-purpose bike/hike pack. It has a very generous 1,480 cubic inches of stuffable space, a single large compartment plus two smaller ones, various cubbies and tie-ons for odds and ends, and an excellent, close-fitting suspension. Even has a rain cover that pulls out of a zippered pocket at the bottom of the pack. It could hold some hiking clothes, hydration bladder, and much more. Plus it has the snug-to-the-back fit thats ideal for mountain biking.
CamelBak's Fourteener ($125) is a tad bigger, with room for day-hike gear and hydration bladders. Two compartments, plus main compartment, good suspension, and lots of room for things. Nice pack.
When I'm out mountain biking for the day, I wear an Ultimate Direction's pack that is no longer made but that is very similar to that company's current Hornet ($95). It can take a HUGE hydration bladder and still has room similar to the REI and CamelBak packs for assorted gear. Less water carried, more room for gear. I like the way the Ultimate Direction pack organizes gear, with a secondary packet that is perfect for keys/energy bars/gloves/hat. A little different pack layout than the other two, and very nicely done. Perfectly suitable for hiking, too.
So there you go.