I've never been a particular fan of neutral focal lengths. Back in my days of 35 mm film, my favorite lenses were the 25, 35 and 85 mm primes. Now, with my Pentax DSLR, I have been using mostly 35, 75 and 130 mm equivalents. Can you see a trend here? I've always bypassed the 40 - 50 mm range, and I've never really missed it. I've always thought it would be difficult for me to produce good images with such focal lengths. One can't resort neither to the dramatic views and superimposed planes of wide angle lenses nor to the detail and subject isolation of longer lenses. All that can be done is careful composition. And that, my friends, demands a certain amount of skill.
Olympus E-P2 + Panasonic Lumix 20/1.7
I've bought a 20 mm lens with the E-P2, which is equivalent to 40 mm - right on the neutral zone. I'm not too happy with the aesthetic quality of the images I've gotten so far. Have to keep trying. Many masters of photography created beautiful works using 50 mm lenses attached to Leica film rangefinders. The problem is certainly not the length of the focal, but the skill of the photographer using it...
PS [Apr 28, 2010]: This interesting article by Andy Westlake discusses what a "standard" lens is supposed to be. According to him, the proper "theoretical" values are 43 mm for full-frame/35 mm film, 29 mm for APS-C and 21 mm for Four Thirds. But the usual convention tends to be: 50 mm, 35 mm and 25 mm respectively.