This Silicon Investor message board post sent along by RIVA3D yesterday mentions John Carmack being a member of NVIDIA's Board. I fired off an email in the wee hours to clarify this, but I see that Billy at Voodoo Extreme managed to catch him at some point yesterday and got an answer already. Here's what Mr. Carmack had to say:

I am on their technical advisory board (NOT board of directors), but I am also on
3dfx's technical advisory board. I give honest opinions to any chip company that
seeks my advice (lately I have been working very closely with ati), but 3dfx and nvidia
have formalized the arrangement.