My motherboard has two ethernet ports: One an NVIDIA, which uses the "forcedeth" driver, and one 3COM, which uses the "3c59x"

I've always used the 3com, as it was better supported when I started using it. But I've recently installed Slack 12, and although it loads both the 3c59x and the forcedeth modules, it only creates eth0, as the NVIDIA port. There's no eth1 for the 3com, and blacklisting the NVIDIA module just results in no networking at all.

Slack's never had a problem with the 3com in previous versions. Any idea why it's not working now?

Might try unloading the 3com module and reload to see what happens.
Might check the changelogs related close in version to your kernel version at www.kernel.org and see if there was a glitch in that version and see if newer releases fixed it.

Brian