I've seen a couple of Asus and MSI MoBos that have integrated 1000Mbs cards. I was really planning to go this route and wire one with a cross over cable from my HTPC to a file server with an Intel gigabit NIC, and utilize all of my major storage and redundancy that the server has for HD streams. The cross over cable would allow skipping the, at the time, somewhat expensive (this was about 6 months ago) 1000Mbs switch, saving me money and increasing the throughput a bit.
I finally had to admit to myself that the draw was just the cool factor of gigabit ethernet, as mathematically it just didn't seem to be necessary. HD streams seem to take a hair over 2.5 MB/s, and 100Mbs network will give almost 12.5MB/s (especially utilizing just a crossover cable to the file server and skipping a switch.)
Also, Gigabit ethernet can almost (95% or so) completely saturate a standard PCI bus, so there is no way to really utilize the bandwidth since the IDE controllers are on the PCI bus as well. IDE drives never sustain any thing close to the 125MB per sec theoretical transfer rates of gigabit ethernet, either.
So, unless you've got a file server with a 64 bit PCI bus along with SCSI drives in some sort of striped array to hit the high transfer rates, AND you need to transfer more than just HD streams all at once, its just not necessary.
Of course, I suppose you could just save streams on the HTPC, and then move them to file server for storage, in a very, very fast way. I doubt that it would be more than twice as fast as a 100Mbs network in most HT setups though. Current gigabit networks for PC's also are somewhat in their infancy, and I have yet to see any reviews of them they state they exceed %75 or so of their theoretical bandwidth.
Let me know if you go for it, though, I'd love to hear the results.