I have a customer with two PIX firewalls. Both currently run 6.35. One is a 501 which cannot be upgraded to 7.x. They would like to upgrade the other firewall, a 515, to 7.x. I am interesting it hearing from anyone that may have a L2L tunnel up between two PIXs where one is running 6.35 and the other 7.x to see if there are any issues with that configuration.