MS Exchange in LTM Arm Mode

From my experience (I'm sure others will correct me if I'm wrong) the only way's you can see the true client IP is to either have the default gateway or use npath routing.

http://support.f5.com/kb/en-us/products/big-ip_ltm/manuals/product/ltm_implementations_guide_10_1/sol_npath.html

both of which require changes to the exchange server, if you only wanted to log the true client IP (which I don't think is what you want) then you can use the x-forwarded-for header, but those are your only options.