Microsoft claims that Windows Vista is the most stable version of their OS yet... so stable that you may not ever see the dreaded BSOD (or Blue Screen of Death). Some of the development staff may have been so nostalgic over their lost Blue Friend that they included a hidden way to raise it from the depths.
Before you begin thinking that this is an awesome trick to play on your co-workers (and it is), there actually is a legitimate use for this "feature". Developers can use this trick to artificially creat a memory dump for testing purposes. But it's much more fun for Linux and Mac evangelists to "demonstrate" how fragile Windows can be.
Once you enable this feature, you'll be able to create a BSOD at any time by pressing a certain key combination on your keyboard.