Wow thats a hell of a lot of LUNs. Sadly 256 is still the limit with version 5. It does sound increadibly high, might have to revise your sizing. What version of Exchange (2010 Im hoping) are you running DAGs and LAGs is this including Mount point LUNs how many users per groups etc. Also how many Exch servers and are their boot disks included??
Might have to spread it out over several Vpshere clusters.