With 'ebtables' you can build yourself a 'brouter' (a Bridging router). This thing will accept packets at one end and spit them right out the other. Some people use this for an IP'less firewall (ie: remotely unhackable).
You could use such a bridge to sniff your traffic, record it, then pass it along as normal.
See:
http://www.linuxjournal.com/article/8172 (example for filtering traffic)
http://freshmeat.net/articles/view/1433/ (example with squid, but still valid)
http://ebtables.sourceforge.net/ (ebtables is like iptables, but at a lower level in the OSI model)