I just picked up a really neat article from Eric Siebert - When Not to Treat VMs like Physical Servers. The basis of the article is common sense really (but common sense is often left at home by us in IT), mostly we can treat Virtual Machines just like Physical Machines but there are a few areas to watch out for:

• Patching – Don’t patch a whole physical machine at once as this might cause huge instantaneous load and impact performance. (Challenging with automated patching tools.)
• Securing – Secure at the Physical and Virtual levels. It does no good to have tight security inside your VM and have weak security outside.
• System Monitoring – Don’t monitor virtual machine hardware, deinstall any hardware management agents from Physical to Virtual migrated images. A key message - because virtual machines boot much faster then physical servers, many monitoring systems will not detect server re-boots as the boot process happens quicker then the monitoring interval. You may find that you need to adjust your polling interval for virtual servers so you can detect the faster re-boots.
• Performance Monitoring – You should always use virtual server specific reporting tools to accurately measure performance on virtual machines.
• Anti-virus – Make sure you install anti-virus software on all your virtual machines the same as physical servers. Again one thing to be careful of is to stagger any on-demand scans and definition updates as to not overwhelm the host server. 
• Backups – It’s OK to backup your virtual machines using traditional operating system backup agents. Always make sure you do not backup too many VMs on a single host at the same time.
• Disk defragging – You should periodically defrag virtual machine disks using traditional operating system tools for maximum performance. However be careful not to defrag a VM that has a snapshot running, doing this can cause the snapshots rapidly grow in size and degrade host performance. As usual do not defrag more then one VM on a host at a single time because of all the excessive disk activity that is causes.

The article is very well written and considered and worth a read.