I realize that you're talking about Linux here, and you're right, but just to drop names, I thought I'd point out OpenBSD. Definitely not a Linux, but likely one of the most well secured Unices out there, including commercial ones.

Yeah, I had forgotten about the BSDs when I posted. I had considered using OpenBSD for my firewall machine, but decided to go with the homogeneous network to save me time and trouble.