That Ken Rockwell quote isn't quite right. Several things to consider:
1. It's Ken Rockwell.
2. What he said about the D50 and D70 doesn't make sense. If the D50 and D70 were both "straight shooters" giving great exposures as set, then why did they underexpose? Also, why does Ken Rockwell say "...which leads less-experianced photographers think they underexpose"? It DOES underexpose. He just knows to use exposure compensation for every situation where he knows the camera will be fooled. It hardly seems fair to say that the D50 and D70 did a great job, while the D40 and D80 do a poorer job. They both need adjustment. KR is just used to using positive compensation rather than negative, and it tricks his brain into thinking, "Wow, this is difficult".
I owned the D50 for a long time, and trust me, the exposure wasn't perfect on that camera. I'd say that's true for every DSLR, from every brand, on the market today, which all take the same approach of underexposing rather than trying to expose well. The only case where this isn't true are the D40 (apparently), the D80, and (especially the) D300, and that's because they use a built-in look-up table inside to try and guess what type of scene you are trying to photograph.
If you're trying to photograph a scene where the top half of the frame is bright, but the bottom half is dark, it assumes that you're shooting the ground on the bottom half (which can be a grassy field, lots of buildings, streets, or anything else) and the sky on the top half. It assumes that you, the photographer, wants the ground to be properly exposed, while the sky can be blown out. To do this, it puts greater emphasis on the bottom half of the frame when metering the light and calculating the exposure. It assumes that you're most interested with whatever is on bottom half. It's actually doing a pretty good job of judging this, as most people really do want the ground to look great. Of course, if you want the sky to be perfect, and you're happy with the bottom half of the photo being a bit too dark, you're out of luck. Exposure compensation will have to be used.