here's what I don't get. The batteries that I have are Energizer
2300mAH jobs. They were sitting in the charger, but the charge light
wasn't on therefore the charger thought they were fully charged. I took
the other pair out of the charger and put them in a mini mag light and
turned it on. It's been on continuously for about 75 minutes now and is
a little yellow but still bright. Is it possible that these particular
batteries have an unusual voltage vs. state of charge curve so both my
digicams think they are fully discharged when they really have 80-90%
charge left? or any other ideas?

The NiMH batteries keep a fairly constant voltage (1.25) until they
become mostly discharged. Your camera has electronics that will
alarm and then shut down once the batteries get to a certain voltage.
This is usually around 1.15 volts or so. A NiMH battery that gets this
low is mostly discharged.

Does deep cycling help or hurt NiMHs? I know some types of batteries
they say to deep cycle them every now and then to make them last longer,
and others it's supposedly bad for them, but I dunno about NiMHs in
particular.

Deep cycling is not needed and could harm a NiMH battery.