Quote Originally Posted by E. von Hoegh View Post
Then why do my meters give different readings, depending upon whether or not light is getting in through the viewfinder?

I've checked this myself and assure you that it is indeed, at least for some cameras, particularly automatic everything models, a metering issue.
Most cameras use the meter reading from just before the shutter is tripped. Or if you are using TTL manual metering. Light travels through the lens and out the eyepeice, but it also does the reverse. Metering systems that measure the light on the ground-glass tend to be more prone to this than ones that use a partial silvered mirror. Some systems use live metering during the exposure (olympus OM, Pentax LX) and some meter after the mirror is raised (minolta MD11, x700).
Especially if you are using manual TTL metering, make sure you meter with your eye over the viewfinder.