My walkaround lens is the kit 18-135. It might not be the best glass or widest aperture, but when I may want to switch from a wide angle to the tightest zoom I can manage at a moments notice, it's the most versatile thing I've got. The nifty fifty is nice when I know I'm going to be the right distance from my subject to have no need for a zoom, but most of the time for a walking lens...focal length trumps all other considerations for me. When the time comes that I want to step up to high end glass, I'll probably be getting the 24-105 L glass...I have a 24-70 for work and am constantly running out of zoom just shy of where I want to be.
**correction** my walk around lens USED to be the kit 18-135. Nooooo!!! by Bill Rush, on Flickr