Thanks, Camaleon (sorry - don't know how to generate the proper characters).
That file includes:
# PCI device 0x10ec:0x8168 (r8169)
SUBSYSTEM=="net", ACTION=="add", DRIVERS=="?*", ATTR{address}=="xx:xx:xx:xx:xx:xx", ATTR{dev_id}=="0x0", ATTR{type}=="1", KERNEL=="eth*", NAME="eth0"
# PCI device 0x10b7:0x9050 (3c59x)
SUBSYSTEM=="net", ACTION=="add", DRIVERS=="?*", ATTR{address}=="xx:xx:xx:xx:xx:xx", ATTR{dev_id}=="0x0", ATTR{type}=="1", KERNEL=="eth*", NAME="eth1"
On further checking, it may be that renaming is acceptable - in /var/log/messages:
Feb 7 04:51:22 puffin kernel: [ 6.122403] 3c59x 0000:03:02.0: PCI INT A -> GSI 18 (level, low) -> IRQ 18
Feb 7 04:51:22 puffin kernel: [ 6.122407] 3c59x: Donald Becker and others.
Feb 7 04:51:22 puffin kernel: [ 6.122411] 0000:03:02.0: 3Com PCI 3c905 Boomerang 100baseTx at 000000000001df00.
Feb 7 04:51:22 puffin kernel: [ 6.148201] Linux agpgart interface v0.103
Feb 7 04:51:22 puffin kernel: [ 6.153035] udev: renamed network interface eth0 to eth1
Feb 7 04:51:22 puffin kernel: [ 6.156559] r8169 Gigabit Ethernet driver 2.3LK-NAPI loaded
Feb 7 04:51:22 puffin kernel: [ 6.156573] r8169 0000:02:00.0: PCI INT A -> GSI 17 (level, low) -> IRQ 17
Feb 7 04:51:22 puffin kernel: [ 6.157040] eth0: RTL8168d/8111d at 0xffffc90000c78000, x:x:x:x:x:x, XID 083000c0 IRQ 32
Feb 7 04:51:22 puffin kernel: [ 6.161239] r8169 0000:02:00.0: firmware: requesting rtl8168d-2.fw
Feb 7 04:51:22 puffin kernel: [ 6.234448] eth0: unable to apply firmware patch
Perhaps the kernel brings eth1 into existence by first establishing it as
eth0, then renaming it to eth1; then bringing the "real" eth0 into existence.
The "unable to apply firmware patch" seems potentially alarming, but it
used to work as a single-interface system. lspci -v indicates both
NICs have "Kernel driver in use".
FWIW the kernel is the amd64 variant.
Any other thoughts? I should probably pull the 2nd NIC to verify that I'm not
delusional, or haven't bunged the configuration somehow, but I'm stumped in
solving this... especially since it is in most respects much like a previous
system. Thanks, all! -F