Netbooks with gigabit ethernet I believe are more often geared towards business users as few home users have gigabit networking.
Do you really want a netbook or do you just want an ultra low-power PC (Intel Atom or otherwise x86-compatible) with gigabit ethernet, like the Asus EeBox B202? There may also be embedded servers running gigabit available. Also, Firewire (IEEE-1394) is generally faster for bulk storage devices like hard drives (if you're looking to be faster) at a slightly additional cost but eSATA is currently the ultimate attachment for the home market.
Currently, I have a box that I term a micro-server that sips at measly 5-7 watts in normal operation (including a hard drive) but unfortunately doesn't have gigabit as it's too old.