confirmation --I can confirm that my Sager 4060 is running with a Realtek 8139. It is NOT running on the PCMCIA bus...
To get it running, the kernel module '8139too' should be loaded.
Are you compiling a kernel from scratch, or adjusting a distro to make your ethernet card work?
If you are re-compiling a kernel, I'd recommend selecting under "Network Device Support" -> "Ethernet (10 or 100Mbit)" -> the "RealTek RTL-8139 PCI Fast Ethernet Adapter support", and underneath that, selecting the options to "Support for uncommon RTL-8139 rev. K" and "Support for older RTL-8129/8130 boards".
If you're running a pre-compiled distro, as root try running this:
prompt# modprobe 8139too
You should then have access to /dev/eth0...
Good luck -- let me know how it goes!