I have a VM running on a cloud server which has a full Linux install, Java, Eclipse and Android dev libraries. That way, I don't have to stick with one, particular laptop or desktop machine when I want to play with it. So long as the machine I'm physically using can handle SSH and X-windows and has a reasonably fast connection to the Internet, I'm good.
Also, most computers can be booted from a Linux Live CD, hit the VM and let me do what I want to do without actually installing anything on said computer. So, if I'm visiting my dad, who's running Windoze, I can still play with my VM.
Finally, using VirtualBox to run WinXP to VPN into the office, on a host machine running Linux, works quite well. If you want to check something on the Internet WITHOUT the VPN trying to route it through the company's proxy server, fire up the browser directly on the host machine, outside of the VM. If you want to hit stuff on the company network, fire up the browser inside the VM.
I foresee a day when more and more people are doing the latter one. Any virus infections on the host machine will have to be very intelligent to infect the VM, and vice versa. And, heaven help you if you ever end up in a situation where you need to log onto multiple VPNs at the same time. Different VMs, different VPNs, one host to rule them all.