Woo hoo! My lastest patch for tcpreplay is done which impliments a pre-processor (tcpprep) and cache function which dramatically improves performance for dual-nic use. Actually, performance seems to match single-nic use which was the goal.
I'm actually pretty stoked about the cache file too- it's super small. For a 900MB tcpdump file, the cache file was only 74K.
Anyways, hopefully I can get Matt to include this into the offical distribution soon. My next feature enchancement will to build some intelligence into tcpprep so it can figure out the server network vs the clients. Should prove useful for replaying in dual-nic mode unknown captures.
Anyways, for those of you who actually read this and care, here's the link to the patches: http://synfin.net/aturner/tcpreplay/