I know it's possible to multiplex ethernet cards, but I don't know if the Apple hardware and software supports it. We have an XServe with OS X Server Panther and it has dual gigabit ethernet. The IT guys are cutting off our second ethernet card unless we can combine the two into a single IP address.
We just assigned both NICs the same IP and the server software didn't complain. But I don't know if that'll work... seems too easy. On the PC side you have to run special networking software and the hardware has to support 802.11q.
Plus, we're having our Dual G4 XServe replaced by a Dual G5 XServe (G4 XServe broke. Sniff.) Hopefully the NICs aren't any different... or if they are, hopefully they support multiplexing.
Just thought I'd ask here real quick before running off to the books and phone support.