The Marvell LAN on my GA-965P-S3 board stopped working all of a sudden after a reboot the other day, and I franticly tried flashing the BIOS & firmware, I undid my overclock, even rewired everything inside. No luck.
The problem was due to the semi-broken sky2 driver... From what I've heard it's been broken for a while (and developers know it). Sky2 seems to work pretty well from my experience.. Apart from this.
Anyways the cause was the new 2.6.21 kernels, which is why it only started happening after a reboot, as I started booting new kernel. Reverting to 2.6.20 (in my case, the FC6 kernel) worked perfectly.
$ /sbin/lspci | grep Ethernet 04:00.0 Ethernet controller: Marvell Technology Group Ltd. 88E8056 PCI-E Gigabit Ethernet Controller (rev 14)