In looking closer at the forged TCP packets I'm getting, I'm wondering if this is some very subtle attack going on.
The sequence I'm seeing is a TCP packet from the forged address with the FINISH flag set. My system then tries to repond to the packet (why? It's not a valid connection to begin with) but the data it sends back contains garbage from previous IP packets, not neccessarily just other TCP packets.
Now, could it be that somewhere along the path some host's NIC is in promiscuous mode and can read the packets, and with a long enough sample of data, might be able to determine information from the partial garbage packets sent back? For instance, I'm seeing my system send back garbage packets with part of my SNMP community string.