I recently upgraded my kernel (mostly due to a recent gcc upgrade) from 2.6.25 to 2.6.27 in Gentoo. When I booted into the new kernel, I received an error of "network interface eth0 does not exist" and to check the kernel to ensure that networking and the driver for the device were enabled.
I was a bit mystified, as things worked just fine in 2.6.25 for my Intel gigabit NIC that comes on the Intel DP965LT. Apparently, it looks like some cards were handled by the e1000 driver until the e1000e driver was considered worthy for production. I switched my driver from e1000 to e1000e, rebooted, and everything was working again.