Like Kevin said, dark photos are not an inherent issue with any DSLR. Some lenses will cause the camera to be somewhat lighter or darker, just as some lenses will produce more colorful photos than others. If you are finding that you're using a lens that is underexposing, you can easily do an exposure compensation, which will bring things up to where you want.
More likely, it's a metering issue. Most DSLRs have 2-3 different ways of metering - this just means how much of the image they look at when determining exposure. Usually they'll look at either the whole frame, the whole frame but give the center the most importance, or "spot" which means they'll only look at the center (or autofocus point). I did a lot of spot metering on my last WDW trip and I did find that this occasionally led to some strange underexposure, but this was definitely the exception, not the rule. In retrospect, I'd probably have been better off leaving it center-weighted most of the time and only occasionally doing spot metering.
As for the D40 limitation, I agree with those that feel that it's a large liability. You can often find very good lenses for good prices in the used market, and if you're paying for an autofocus lens, it would be nice if that lens actually autofocused! I'm all for manual focus, but with no split-prism or microprism assist in the viewfinder, MF can be a bit tricky - even if the camera does confirm focus (which I don't think that the Nikons do, but I could be mistaken.)
For the Pentaxes, the K100D is definitely the way to go at the moment, with such a small price difference between it and the K110D. The only difference is image stabilization, but that's a pretty important one.
Anyway, if you're really leery, you can check your local yellow pages to see if anyone rents equipment. It can be a little on the pricey side and selection will be limited (probably C/N only and maybe not even current models). For example, a local shop gets $95 a
day for a Canon XT with 18-55mm and a 512m card, and that's the only DSLR. There are discounts for longer use (7 days is charged for only 4, etc), but still, a week's rental and you're getting pretty close to having bought the darn thing.

I'm sure some other places have more reasonable prices.
You may also check out the return policies of your local camera store or big box store. You can always purchase one of the cameras and if you're not happy with it, return it before the time is up. If you have enough time, you might even be able to squeeze a Disney trip in there.
