You see it everywhere these days. People boot their computer, and grub or lilo comes up with a list including (but not limited to) Windows, Linux, and that bios pong hack. But what's the point? You can only use one at a time. Many people would suggest getting a second computer if you see this as a problem, but this would mean that you need two separate computer cases. My solution is much more 1337.
The solution isn't even all that complicated - anyone could do it. All you need is a beefy power supply, a nano-ITX form factor motherboard, an extra hard drive and an ethernet card, and you're good to go! Slap that board in the space where your PCI expansion slots are (you know you're not using them), hook the built in ethernet on the nano-ITX board into the new ethernet adapter that you just installed via a nice 4-inch crossover cable, and you now have two computers in one.
Install your favourite flavour of Linux on the second hard drive, and use your windows side to ssh into the Linux side. Be sure to install Cygwin to get the X11 server on your windows side, so you can still use all the graphical programs available on the Linux side. You could even use virtual desktops available on either side to use the other side as a normal computer as well, since the other computer is basically just another box on your LAN. Alternately, you could get a small VGA cable splitter, and somehow hook up the switch to the front of your computer. Then you could get around the issue of running your OSs as picture-in-picture, and still use the dedicated video hardware on both sides! If you do this, don't forget to use the same switch on the front of your case to split the USB connections for your mouse and keyboard, too.
For the truly 1337, instead of having the master/slave setup for ethernet that I described above, you could get a switch or router instead of the ethernet card and crossover cable combo. Strip the casing off of that sucker, stick it in the remaining space in the front of your case (under the drive bays), and voila - you are now the centrepiece of any LAN gaming extravaganza, and now you don't need to start up the master side of your computer to give the slave side internet access.
This is a small taste of what you can truly do with your computer hardware. I believe that this is the true meaning of dual-booting - two OSs, two sets of hardware, two boots on startup. When you put two OSs on your machine, and only load one of them, you're only a half-booting wussy.