Virtualization is one of my favorite topics. I often write about how it's a silver-bullet solution to a lot of problems. For example, I just mentioned yesterday how, if the vendor of your favorite software isn't patching that software to run more smoothly on Windows Vista and is instead releasing a whole new version for a price your not yet ready to pay, then maybe running that software in an XP-based virtual machine guest on top of a Windows Vista running-on-the-bare-metal host is the way to go. At least temporarily. You may not get the performance you want (VMs tend to run slower than running on bare metal). But that might not be an issue for you.
One problem with the VM approach however is that the software industry still hasn't quite figured out how to ease the licensing restrictions around using the software you buy on virtual machines. For example, if I have a copy of Windows Vista on one system and I want to de-commission that system and re-use my license in a virtual machine on a new system, it's not that simple. As Ed Bott points out, the rules for Vista have been modified to allow for this as long as the original version of Vista that you're moving was a retail version. If it's an OEM version, no can do. In fact, chances are you won't even have the DVDs that you'd need to do a clean install -- only recovery media that works differently.
Some of the same confusion exists around application software. One fear software companies have is that people will use virtual machine technology to build little multi-user mainframes (sort of like Citrix) where anybody can use the built-in remote access technology to simply take control of a VM on a computer and run their software remotely, that way. If XYZ software company says it's OK for me to copy its software to as many VMs as I want, but only one one system, technically, I could let lots of users access that one system remotely and abuse the license.
Anyway, at some point, virtualization technologies and software licensing will come to loggerheads. This whiteboard video attempts to make that point.