Another option is a Linux netbook configured with VLANs to get around having only a single NIC. Then if you lock yourself out via the network, you have a keyboard and monitor built-in and don't have to go find the serial cable in your stuff-you-hoped-you'd-never-have-to-use-again box.
[1] http://www.portwell.com/products/ca.asp#SOHO [2] http://www.lannerinc.com/Network_Application_Platforms/x86_N...
(obGripe. wish these companies would make it easier to buy stuff online...)
So I doubt the ability of this platform to even be able to route packets amongst the four interfaces at full speed, let alone filter the packets at the same time. It will, however, do better than the existing Soekris products.