We have a fairly large Windows visualized farm on VMWare. A few hundred servers we've migrated over or built on VMWare over the last 3 years or so.
However, in the last 12 months, we've gone from having 2 IFLs to play with, to more than 20, running well in excess of 400 Suse Linux servers on the Mainframe. We expect to at least double that this year, and ANY server that CAN run on Suse is being migrated to Suse.
Windows needs 1 license per virtual server in most cases (OS excluded), where most IBM license is by the processor underneath. A P6+ 4 core processor is 480PVUs worth of license. An IFL is 120PVUs, and can run 4-6 times the number of guest servers.
You can now get a mini-mainframe (Buseinss class z10), with 2 starter IFLs for under $200,000. That can run 40-60 servers easy. The same capacity on a 570 or 590 platform will cost about the same, if not a bit less (we pay about $80 per fully loaded VMWare chassis), and it will also run 40-60 guests. However, licensing costs between them, for things like WebSphere, are about 8 times higher on the VMWare side, and on top of that you need the VMWare licenses themselves, plus the OS. A z10 BC can hold a whole lot of IFLs, a few ZIIPs and ZAAPs too. If you;re visualizing a few hundred web and Java servers, DB2 or PGSQL/MySQL databases, WebSphere or MQ, then z/VM linux on a z10 IFL is by FAR the cheapest way to go. We've also seen cost reductions in Oracle pricing, VRU services, and more.
The single binary image model is far superior to image independent VM guests (patch one system to patch a hundred). Operational issues are lessened by not running Windows, no one writes viruses compiled for OS390X hardware (since you'd actually need access to a mainframe or Hercules VM server to do it), and LPAR replication between 2 chassis is easy (and you don't have to license the second chassis).
If you look at IBM list pricing for the IFLs, ZIIPs, and Chassis, it sounds like a bad deal, but consider, NO ONE pays list price for IBM mainframe hardware (we pay about half).
Is this a solution for an SME, with 30-50 servers? no, but then honestly nether is a true VMWare infrastructure...