A friend of mine's shop is considering virtualizing several of their servers, including their Exchange servers (2007) and their BlackBerry-Exchange gateway systems.
The systems support roughly 150 mailboxes now, with limited growth projected over the next few years. The servers are not currently heavily utilized, but the company is investing in upgrades for other portions of their infrastructure, so this is an appropriate time to upgrade the exchange box as well. Storage space seems to be their biggest issue now; they'll be moving to a SAN for their storage if they go this way.
What guidelines would you use when considering whether a virtualized environment makes sense for exchange? Are there significant issues around Microsoft's support policies? At what sort of load levels does it no longer make sense to virtualize?