I use a Highway USB in the Mediator PCI 4000 and it works without problems. As flash ROM I use an eFlash4000, though. The Poseidon software works fine from the ROM, I can boot frmo USB drives.
AFAIK none of the threats has been made true yet, neither have Elbox changed the Mediator to reject E3B cards nor has Chris Hodges changed the Poseidon software to reject the Spider.
Bye,
Thomas