When upgrading to RHL 6.1 over NFS from a machine on the local subnet (with no default gateway defined), only the first network card is probed for the NFS server. So, if your NFS server is accessed through eth1, the installer won't be able to find it. Workaround: plug the cable from eth1 into eth0.
This issue has been assigned to a developer for further action.
*** Bug 5633 has been marked as a duplicate of this bug. *** While installing a dual P6/200 with an on-board 10Mbit pcnet32 and an eepro100 card (eth0 and eth1 respectively recognized by the installer), installation assumed eth0 as the installation device (which wasn't hooked up to a switch at the time... had to move over the wire from the eepro100). Selectable when num_eth > 1 would be nice :)
This issue has been added to a feature request list.
Fixed for next release. Sorry it took so long.