Well Freind,
This scenario is really different from those where i did before. But it can be done. as to virtualizing a physical host is not a rocket science. it can be done easily. only for a safer side you must check with the 2003 iis server where exchange also playing a big game. you must take care of all the prerequisite required by exchange. Also must take care about the shared disk which are shared between host, also need to be shared in to that ESXi host as RDM if any (Most probably it will be there due to exchange server). Else there is no any issue for this. yeah. i cant say how much troubleshooting you need in your case. depends on the complication of your network and infrastructure setup. 
All the best,
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Anjani Kumar
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VMware, Windows
Consulant
Email ID : Anjani.kumar@vmwareminds.com
Website : http://www.Vmwareminds.com