In-camera meters measure the light reflected from the subject. It's all they can do. The trouble is that different subjects reflect different amounts of light. A white wedding dress will reflect a lot more than a black cat. But the camera can't tell white from black (that's a cognitive thing, not a measurement) so it has to assume everything is a sort of mid-grey. Which is why, unless you override the exposure, it'll give you grey wedding dresses and grey cats. And this is why a simple handheld meter and a pair of feet can deliver exposure accuracy that no camera can, be it an EOS 1Ds Mk III or a Nikon D3x...
What you need is an 'incident' light reading, where you measure the amount of light falling on the subject, not the amount it's reflecting. This ignores the brightness of the subject itself and delivers pictures where wedding dresses do come out white, and black cats really are black.
So the next time a camera maker trumpets an all-new and even more 'intelligent' multi-pattern metering system, just remember that it's still measuring the wrong thing.
