I get a lot of questions about virtualization and really have few answers. At a high level it's not hard to see some opportunities in an enterprise where it makes sense, perhaps the most obvious being hardware consolidation, followed by virtual PC environments for testing. I'm sure there are other valid scenarios too. But when do you virtualize your db server? I can see cases where you have legacy stuff running on an NT4 box with SQL 7 that you don't want to mess with, but would you really want to virtualize your primary db server? You know that 4-8 processor 16g monster you're relying on?
I'm not anti-VM, if nothing else I assume that whatever performanc hit may be involved can just be offset by adding one more processor, 1g more RAM, etc, etc. For me, what I really want is OS consolidation - I want fewer servers to patch. Running VM's doesn't reduce the patching time. Maybe now that 64 bit SQL is here and we get a degree of resource throttling in SQL 2008 it will make more sense than in has in the past to run multiple instances of SQL Server, though if I could have it all I'd want SQL consolidation too, fewer instances to patch.
I think one big win for virtualization is that it might take the fear out of purchasing, especially on the database side. Ask a DBA for their recommendations and I'd be surprised that even a server that will host the payroll database for a 2 person company wouldn't be the proverbial mac-daddy server - we're just too conservative to order what might be the minimum hardware, and so we wind up with very under utilized servers. While those 'wasted' cpu cycles aren't great, the real pain is a combination of the wasted capital funds plus the electrical bill to run it. With a good VM set up companies can start with a server that sized more on the conservative side and grow if needed. I'd bet that approach alone could add up to a staggering savings across the US.
Information Week had a nice article on Virtualizations Promise and Problems that is worth reading, among other things it mentions virtualizing IO and memory!
Maybe the last question - to which I don't have answer either - is what value does virtualization have for smaller companies, the ones with a full of servers or less?