I'm surprised no one has mentioned the new Canon 15-85mm lens. I recently purchased it and found it to be my ideal travel lens. The qualitylens is tack sharp (comparable to the 17-55), well-built (supposedly better than the 17-55 in this regard), has an immensely useful range, and is about 20% lighter than the 17-55. It is also about 25% cheaper than the 17-55mm. As one of Canon's newest lenses, the IS and USM focusing are top-notch.
The only con (other than arguably price), and this could be a big one depending on what you shoot, is the variable max aperture. This is not a good night-time lens. It's IS makes it fine for shooting churches and other indoor architecture, but don't expect to get good shots of people at a bar. It also won't get you amazing depth of field (although the bokeh is still nice). Frankly, I consider these minor inconveniences: for night-time or cliched flower shots I just swap out my 35mm prime lens.
I believe the 15-85mm was designed in every way as a general walk-around lens, which makes it ideal for the types of vacation shots I take.