it is not exactly clear to me what triggers this, but it's absolutely reproducible (big Thank You to SynrG from #eeepc for telling me how to fix this). Ethernet works fine when booting the ASUS 1001P pinetrail netbook from one of the live CD, but my legacy karmic system (lucid kernel!) on HD booted from USB will not show the ethernet device in lspci or ifconfig unless giving the kernel the boot param eeepc_laptop.hotplug_disabled=1. Upstream knows about this and has committed a fix, although that needs to be broadened to include the 1001P as well, not only the 1005P, it seems. Please look through http://bugzilla.kernel.org/show_bug.cgi?id=14570, especially comment 31 and make up your mind what's happening here. I don't understand it ;-)
I think we should have a fix for this in lucid and possibly in karmic as well. These are cheap and popular devices that many newcomers will use for taking Ubuntu for a test drive.