Here's my take on Virtualizing servers:
Prove to me that your server isn't a good fit for virtualizing, and then I'll believe you. There are always the normal exceptions that have an outdated PCI card (phone systems are the prime example here), but for the most part, I fail to see why you wouldn't virtualize EVERYTHING else.
Take the following scenario:
You have a server sitting on Hardware. That hardware tanks (bad MB, bad Proc, etc) as will happen in 100% of servers before they are canned. You now have to wait for parts, rebuild, or worse, start over from scratch.
Had you though ahead and had your server sitting on Virtual Hardware, when that hardware tanked, worse case scenario (aka, your too cheap for a real solution), you load up another piece of hardware with ESX, crank it up, and copy over your VMDK files, and WOOHOO! You're up and running withing a few hours instead of a few days. Best case, you had Fault Tolerance turned on, and no one even knew it happened in the first place.
- Now you're the hero -
Just my viewpoint, but what do I know. It's not like I'm certified or anything... Oh wait, I am :-)
Here's a book that should help out:
VCP VMware Certified Professional on vSphere 4 Study Guide: Exam VCP-410