Note that the little Pentax K100D with a pancake prime is actually lighter than the M8 with a lens. The DP1 IS a great deal lighter (half the weight). Of course the M8 has a huge advantage in image quality and build quality (the K100D is a $450 consumer SLR, and the M8 is a $5000 Leica), and there are more than three lenses available - Pentax has 21, 40 and 70 mm pancake lenses that are, if anything, smaller than Leica rangefinder lenses, but anything else will be quite a bit bigger. For compactness, there are always the (frightfully expensive) Leica Tri-Elmar lenses for the M8 - 3 focal lengths in one fairly small lens.
In the days before autofocus, there were VERY small zoom lenses for various SLRs, some of which were optically quite good. Their secret was that they had shorter zoom ranges - 35-70 was common, instead of today's 28-105. Nikon actually still makes (or at least B+H still lists) a 35-70 f3.3-4.5 manual focus lens that is less than 2.5 inches long - of course its compactness advantage is destroyed because it won't meter on anything less than a D200, and it won't even focus on a D40 or D40x.
That lens is exactly the same size as the 28-35-50 Tri-Elmar, and I wouldn't even be surprised if the optical designs are somewhat similar (of course the Leica will be much better made, with superior glass). The present cheapie 18-55 f3.5-5.6 is not the same class of lens (the 18-70 f 3.5-4.5 is, but it's nearly twice the size and weight). Even the 18-55 is substantially larger than the old 35-70 (although similar in weight). Why can't Nikon bring that lens back in an AF-S VR version - perhaps as a 24-50, so the focal lengths are a better fit for digital sensors?
Better yet, build a non-SLR camera with the 24-50, or even an 18-50 built in - well under a pound, D40x sensor, nice optics, RAW and VR. That lens would have a zoom range from somewhat wide to a portrait lens - if you want telephoto, carry your SLR... I bet Nikon would sell quite a few of those cameras for $700-$900 to owners of D200s, 5Ds, 1D series cameras and D2xs, looking for something to carry when they don't want to bring the big cameras.
-Dan