I'm surprised nobody has mentioned Osprey's convertibles. I can't recommend them more highly:
Wheeled convertible carry-on with detachable daypack:
http://www.ospreypacks.com/Packs/MeridianSeries/
Wheeled convertible (too big for carry-on) no daypack:
http://www.ospreypacks.com/Packs/SojournSeries/
They've also got travel packs that have detachable daypacks (the "Waypoint" series).
Osprey is great quality, comparable to Patagonia if not better.