Q: On linux I can see the /dev/ttyUSB* devices for my Icarus FPGAs, but
cgminer can't mine on them

A: Make sure you have the required priviledges to access the /dev/ttyUSB* devices:

 sudo ls -las /dev/ttyUSB*
will give output like:
 0 crw-rw---- 1 root dialout 188, 0 2012-09-11 13:49 /dev/ttyUSB0

This means your account must have the group 'dialout' or root privileges

To permanently give your account the 'dialout' group:
 sudo usermod -G dialout -a `whoami`
Then logout and back in again