It conflicts with r8169.ko inobviously.

The whole mess looks like this:
- r8169.ko doesn't work for all of Realtek 8111/8168/8169 mutations
- r8168.ko works with some of the chips r8169.ko doesn't
- r8168.ko also works with many chips r8169.ko works with
- r8169.ko is provided by kernel-image package (thus default)
- r8168.ko is provided by kernel-modules-r8168 package (optional)
- kernel-modules-r8168 package requires r8168-blacklist package
- r8168-blacklist package is a one-liner that blacklists r8169.ko
- STAGE1_KMODULES wouldn't include r8168 (std-def) or rtl8168 (led-ws)
- sub.in/stage1/modules would mention r8168.ko (m-p-d: r8169.ko)

So a LiveCD built with use/kernel/net might work with RTL8111/8110
just fine when booted live but fail to automatically load the module
when installed onto hard drive; manual modprobe r8169 would work though.
NB: some of the chips (those available to me) would work just fine
both ways -- this has contributed to fixing this *that* late.

Bottom line:

  do not install backup/kludge drivers overriding main ones by default!

Thanks sem@ for providing the crucial hint.