Proper Way To Connect Two 3400's To Two Cisco 6509's

I would second that. We have run for about 10 years now with redundant F5 Load balancers on Cisco switches with one load balancer connected to one switch and the standby load balancer on the other switch. All of our server switches are deployed with a L2 port-channel (2G) trunk between them. We don't do trunking on the F5 ports. We do one-armed load balancing with SNAT, and just have the one interface in a vlan dedicated to F5 virtual servers, and let the load balancer route thru the cisco network to the member nodes. That way only load balanced traffic goes thru the load balancer. I'd be very very careful about spanning tree and the trunking if both the 3400 bigips and the Cisco 6509's are participating in that, could lead to some rather challenging problems....