If this is actually the case, 2 nics per workstation with one internal and one external. then your outside IT guy is a moron. The sheer waste on complexity of design of dual homed machines would warrant a firing squad, not to mention that unless the 'Internet' network connection was well firewalled these dual homed machines are going to be the source of the virii and if you've got a decent firewall then you don't really need to be dual homed(machines protected). Even if you were taking a security/confidentiality angle, the dual homed machines could all route traffic, virii and sensitive data and all, between the private and public networks.

I can hear it now, "I've got the server behind a firewall of multi-tiered Windows XP desktops."

That's the goofiest thing I've heard in quite a while.

Nuditarian is right. Separate ethernet ports will provide little to no security advantage. If you take one computer on the private network and add a second port to connect it to the web, that computer can easily act as a bridge between the two, so your private network is no longer as private as if you just used a router with port forwarding turned on. Even then, if the internet connected PC got infected with a virus (no known Mac viruses exist) then it could pass it on to others on the network.

Also, any extra bandwith provided by a second port will go unused since the Minis hard drive will not keep up.

There's been a lot of discussion about this, but I have yet to see one good reason to add a second ethernet port. ;)