In my post/rant about IM clients, I mentioned that I was running a Ubuntu instalation in a VM instance on my MacBook. I am in fact, not crazy, and I'd even go so far as to recomend this experience for most other people. Here's why:
First off, virtual machines let you save state in snapshots. So, as a general practice make a backup snapshot when you get your computer set up (which you save for safe keeping), and then again at regular intervals (every week, say), and then a third one just in case before you change any setting that might screw things up. That way, if things get really bad, you have a known good setup, something working that's no more than 7 days out of date, and protection against botching your system in an upgrade. This isn't as backing up your data, (which you should also do) but it's important to do anyway.
The second great thing about virtual machines is that they let you sandbox the operating system. But you like your operating system sans sand? Me too. Conventually we run our operating systems "on the metal" and our operating system is in charge managing all the hardware interactions, but in virtualized instances the VM software does all this for you, and the "guest" operating system runs in an isolated enviroment. What this means is that you can move a virtual machine from one computer to another (mine's 6.3 gigabytes, I could put it on a flash stick!) without any problems. Also, if you have some sort of driver problem on your host operating system, the VM isn't subject to that, and you won't get intsability from crapy drivers (exactly). And if you're using the VM for testing and you manage to screw up something crucial the VM is the only thing that crashes.
Not to mention that you can generally start, stop, and pause virtual machines at will, so say you're working on things in a virtualized linux, but need to run OS X system updates, you can puase your work in the VM update the system, restart and then unpause the VM and be right where you stoped.
VMs have become more popular/prevelent in the last several years as Macs have started running on the same x86/"intel" hardware that PCs have been running on for years. If Macintosh hardware is just pretty looking PCs with only one mouse button, it makes it easy (and tempting) to want to virtualize OSes on desktops, particularly as for people switching from Windows to OS X, who either need to use their existing software or who just want something familar near by. And the great news is that since virtualization has been used for years in servers, the programers are pretty good at writing the software.
So it seems to me, that VMs might be the way that we all interact with our desktop computers in a few years. There are a lot of useability/backup benefits, not to mention the portability ability, and it could also improve operation, depending on who gets stuck with managing the metal/hardware. I think the possibilities are pretty endless.
Onward and Upward!