I just experienced my first ever system lockup. No mouse, no keyboard
with an Iceape screen open.
I had saved a posting regarding SYSRQ and I tried Alt-Sysrq k then
Alt-Sysrq i and the Alt-Sysrq b with no response.
In desperation I crashed and rebooted the system. I am using
linux-image 2.6.21-2-686. When the system came up I entered grep SYSRQ
/boot/config-$(uname -r) and got the response CONFIG_MAGIC-SYSRQ=y
So its there. Either I did something wrong or it doesn't work with the
problem I encountered.