2009-10-06.

So:  why does this
	echo b > /proc/sysrq-trigger
no longer work as a way to hard-reboot the computer?

It turns out that CONFIG_MAGIC_SYSREQ, under the
'Kernel hacking' category of kernel config, is needed
for this.  
Interestingly, it also turns out that enabling this
option has the main (at least, earlier-implemented) 
effect of making certain key-combinations do special
things -- potentially very useful, but not always what
one might like the cats to do to the desktop machine.

http://en.wikipedia.org/wiki/Magic_SysRq_key

As a sysctl.conf setting, kernel.sysrq = 0 will disable
this feature even if it's configured.  So
	echo 1 >/proc/sys/kernel/sysrq
will temporarily enable it.

There exists a sysreqd program that can be set to listen
to (default) port tcp 4094, to give sysrq commands 
with simple plaintext password control.

INTERESTING (bearing in mind annoying consoles after 
killing an nvidia-based X session):  a graphical program
`has no chance to restore text mode' (if crashed),
but a readable text console can be restored with the
	textmode
and
	reset
commands, the former being within the svgalib package.
I'd noticed that reset didn't work in these cases, 
and I see that textmode isn't on our gentoo systems, 
so I'll install it.

There's no mention of the possibility of allowing only
the /proc-based interface without allowing the keyboard
to do any special things.

To use the keyboard-based commands, hold the Alt and 
SysReq keys together, then press the further key that
sends the command-character.  AltGr may be used instead
of Alt in some cases, and keyboards without SysRq will 
generally have a Print key that can be used instead.

The commands for /proc/sysreq-trigger, or the sysreqd
or keyboard (assuming a QWERTY layout) are:
 0-9	set console log-level
  b	hard reboot
  c	reboot kexec and give crashdump
  d	display currently held locks
  e	SIGTERM to all processes other than pid=1
  f	call oom_kill to choose and kill a process
  g	switch back to kernel's framebuffer console (after kernel modesetting)
  h	give help info
  i	SIGKILL to all processes other than pid=1
  k	kill all processes on the current console, inc. X and svgalib
  l	display stack backtrace of currently executing tasks
  m	display memory information
  n	reset nice-level of all high-priority and realtime tasks
  o	hard power-off
  p	display state of registers and flags
  q	display all active high-resolutoin timers and clock-sources
  r	keyboard from raw mode (as with X11) to XLATE mode (?)
  s	sync all mounted filesystems
  t	display details of all tasks
  u	remount all filesystems read-only
  v	display Voyager SMP processor information
  w	display blocked (D-state) tasks

Pretty useful!