http://www.serialata.org/esata.asp
people might think it would get congested, but mostly only on a Burst, and even then, probably not, as the PCI spec tells you it can handle 155Mega
Byte/s sustained, and its also DMA'ed so that to helps a lot...
why not just make your linux semi NAS into the gigabit router if its already powered anyway,or even simpler, if you only have one machine able to benefit by a direct gig iSCSI connection , just use a simple extra gigabit card x-over directly to the desktop machine and leave the 10/100 to be used by the slower wireless/powerline machines.
an idea for the more advanced *nix readers to play with anyway....