for me, a "walkabout" lens (again, depending on how you define it) should be the lens that you have no qualms lugging it around on your camera for the best part of the day when you are walking about. this should mean that it is light enough to be comfortable on your neck (if you're using a neck strap).
but more importantly, it should enable you to shoot whatever you fancy along the way. as long as it allows you to capture the images you had envisaged at that moment just before you released the shutter, that to me is the best "walkabout" lens.
for me, when i was still using the d200, my "walkabout" lens was the able 18-70mm. the images are sharp and the distortion is "acceptable" for a lens of that price range. besides, you wouldn't feel its weight even if you had to walk around the whole day with it.
if i didn't mind the hassle or have the time to change lenses, i'd pack a couple more lenses. but that would depend on what i'm shooting. generally, my af 50/f1.4d and af 85/f1.8d would be in my bag while the af 35/f2.0d would be at the ready on my d200.
but now that i've sold my d200 and shooting with d700, my af 135/f2.0dc has suddenly found its way into my shooting arsenal again. again, the af 50/f1.4d is sitting ready on the fx body. i'm also rediscovering the joy of my 24/f2.8 ais.
but your preference may vary.