During today's hardware testing, I learned that the SysRq button actually does stuff!
The appropriate combination of key presses can send signals to the Linux kernel to make it do things, particularly in the event of crashes or freezes.
The magic SysRq key is a key combination understood by the Linux kernel, which allows the user to perform various low level commands regardless of the system's state. It is often used to recover from freezes, or to reboot a computer without corrupting the filesystem.
The option to enable this needs to be set when the kernel is compiled and then also enabled. On my Fedora 14, it appears to be disabled by default (/proc/sys/kernel/sysrq is 0). When I have a bit of time, I'll try enabling it to see if it works.