Digital photography

Any advice on a decent walkabout lens for my 550D? I'd love to go with something like the 24-70mm f/2.8 USM L series, if I can find a decent second hand one. I'm still a little fuzzy on the crop/full frame thing when it comes to lens choice.

That would give you a very unbalanced, lens-heavy camera and a less-than-ideal field-of-view, as 24mm becomes 38.4mm when you take into the 1.6 crop factor of that body. That may be wide enough for you most of the time, but probably not all the time. I'm a Nikon so I'm not sure what else Canon offer but they probably do a fast normal zoom in the 17/18-50/55mm range, which would give you the same view as that 24-70 would on a full frame body. If not, I used and was very happy with Sigma's 18-50 2.8 when I had it.