pocket firewall router

Does anyone make a dedicated iptables microcomputer? You know,
something with a couple NICs and enough of a linux os to run iptables
so I could run it in-line to a box and have a full NAT firewall
router I could carry around in my pocket like a RAM stick. I've been
looking at otions like Gumstix, Blackdog, and picotux, but I'm
wondering if there isn't something already out there I haven't
run across.