If the answer is the former, then Eagle Creek is the way to go. They make a bag called the Switchbackprobably what you sawthat normally acts like a regular roll-around travel suitcase. But, it also has internal shoulder straps and a waist belt that can easily pop out and be used for long treks to the hotel and so on. Not really great for extended hiking, but perfect for general travel. The Switchback comes in two sizes: the 26-inch-tall version ($285; www.eaglecreek.com) and the slightly smaller 22-inch ($250). The 26er is not the biggest bag in the world, so frugal packing will be a necessity. But it should be adequate.
If you want something that will spend most of its time on your back, take a look at another Eagle Creek product called the New World Journey ($275). Without wheels, you heft it on your back or with a side handle. After that the list gets short. Several companies that used to make good travel-oriented packs, such as Lowe Alpine, have bailed on the category, probably due to the Eagle Creek juggernaut. But in many cases an ordinary backpack works fine. Airlines supposedly are fussy about the straps and such, but I've never seen anyone challenged on it. And you can always take a large, lightweight duffel pack to stuff the backpack in when it's checked as luggage. Gregory's Shasta ($249; www.gregorypacks.com) is a big, clean pack that's a wonderful backpack but also would make a terrific travel pack.