Are you planning to put CSMs in both data centers or a single data center?
If CSM will be at one site (site A) only then
You need to use source NAT on CSM (Assumption: its in Site A) to make sure that the return traffic from servers in "site B"
can go back to the client through CSM at "Site A"
If both Data centers will have their own CSM modules then the best approach is to use GSS or any any Geo redundant setup that can check the load/health/availability/proximity of the VIPs (virtual ips configured on each CSM) in each data center and direct the clients intelligently to the appropriate data center.
Syed Iftekhar Ahmed