With linux, is there any way to compromise the USB stick used for the air gap? AFAIK the Stuxnet virus was originally spread via USB stick, however I reckon that it involved Windows machines that are known to execute files on USB sticks.
[0]: http://ps3wiki.lan.st/index.php?title=PSJailbreak_Exploit_Re...
You could get all stuxnet and exploit the various applications (such as the components that inspect zipped content), but the transport itself is a simple file copy over a bitstream. You could do the same thing with kermit and uuencode a bit more easily.
I've heard of using serial lines/modems with the appropriate tx->rx cut, but I don't know if it would actually work for ethernet (maybe 10BaseT only?)
Half-duplex, 100mbit, ignore link. I suppose it can be done?....