I haven't done so yet. It's an intermittent fault which only appears to show after rebooting, and is mostly resolved by a cmos reset. Cheers for putting a button for that on the I/O panel by the way.
Some research online shows this to be a known fault with some UD5s, along with the standard fix of cmos reset. Unplugging and leaving the cmos battery out for half an hour was suggested as a permanent fix, but didn't work as such for me. It seems to be much less common than the cold boot issue though, and even with the two combined it seems a vanishingly small fraction of X58 UD5s are affected.
I'm trying to decide if it's worth shipping it off, given there's a reasonable chance it wont show up under testing, will feature about a weeks downtime which I really don't want (longer if it's deemed easier to replace the board than to repair), and that the shipping process probably costs more than a second hand nic.
I'm certainly annoyed by this, but a new ethernet card is probably the way to go. The board wasn't repaired incidentally, instead returned as no fault found. There was considerable photographic evidence supporting this so I have no problems with the previous rma and remain pleased with the service. It's unfortunate that the fault slipped through the net though.