I have a Fuji FinePix 200A working good
on my Pegasos. I also tested a Concord
Duo 2000 with success. I imagine most
Poseidon supported USB interfaces would
work fine with these cameras as well.
These cameras simply mount a drive volume
on the screen when inserted and you can
copy to and from them just like a pen drive
or similar device. They are recognized by
Poseidon as MSD. (mass storage devices)
Here is an example picture from the Fuji:
http://www.telusplanet.net/public/samel/pcengine.jpg