"Xen is free, and virtualizing servers will free up existing capacity to support new workloads," says Roussain.
Erm, no, it is not free. At least not the version that will be of much use in a corporation.
XenServer 5 Express is free. It is akin to VMware Server, or at very best VMware ESX Foundation.
XenServer 5 Enterprise retails for $2500 USD for up to 4 sockets. That's a tidy sum cheaper than ESX 3 Enterprise-- less than half-- but it currently cannot do DRS or Storage VMotion so it's not an even comparison today.