Where can I find a one-page definitive & reliable reference for basic
default emacs keybindings?

By definitive I mean complete and accurate.  (I don't mean all
commands, perhaps just the fifty most-commonly used commands, with a
focus on text editing commands.)  By reliable I mean somewhere that is
semi-official in the (x)emacs world, and doesn't look like it's going
to go away soon--e.g. gnu.org, emacs.org or similar.

I'm basically looking for something like
http://www.unix.org.ua/orelly/unix/unixnut/ch07_03.htm except legal--I
don't think this site is actually allowed to post this content. 
Something that's close  is
http://www.gnu.org/software/emacs/manual/html_node/Moving-Point.html,
except this only covers commands to do with moving the cursor, not
with commands like M-q (fill-paragraph).

I'd prefer a web page, but pdfs are okay too.  (As long as the URL
doesn't look like it's going to disappear soon.)

Hello Mjsx,

Let me give you several on line references as well as describe the
capabilities of emacs to generate information about itself.

After starting emacs, the command (M-X for Meta-X, or escape-X)
  M-X describe-bindings
is THE definitive listing of all keyboard bindings (commands) and
works for whatever buffer / major mode that is currently active. You
may want to run this with a C program active for example to get the
additional commands available while editing C program files. This
should work in both GNU emacs and xemacs. A nicely formatted version
of the default version of this should be on your system as refcard.ps,
typically in a location like
  /usr/share/emacs/etc/refcard.ps
or use
  find /usr/share/emacs -name "*refcard*" -print
to help locate it. I also found
  http://www.delorie.com/gnu/docs/elisp-manual-21/elisp_334.html
to be helpful in describing what describe-bindings does and how to interpret it.

For specific on line summaries of Emacs commands, I suggest sites such as:
  http://www.fnal.gov/docs/products/emacs/refcard.html
a reference card for Emacs version 19 hosted at Fermilab.
  http://www.cs.rutgers.edu/LCSR-Computing/some-docs/emacs-chart.html
a two column reference card at Rutgers University.
  http://www.blackie.dk/emacs/home-page/refcard.html
files from an emacs reference book. You may be able to copy / paste
the output of describe-bindings into one of these to make a nicely
formatted version.
  http://docs.cs.byu.edu/docs/emacs_intro/index.php
an introduction to emacs from Brigham Young University. Includes
topical subsets of commands (as well as explanations).

There are several subsets available as you have noted as well. Use one
of the phrase below if you think these lists are too long.

As a more advanced listing of options available in emacs, try
  M-X list-options
This is likely far more than what you want to customize, but it may be
handy in helping to diagnose problems that a user might get into.

To search for more options, try phrases like
  emacs reference card
  emacs describe bindings
  emacs quick keyboard reference card

If you need additional informatino related to emacs, don't hesistate
to make a clarification request.

  --Maniac

I don't consider any of the on-line references offered to be
"semi-official"; going by the URLs, they also seem likely to disappear
at some point.

Here is one:
This reference card lists some of the most common commands for the
Emacs text editor under Unix.

<A href="http://www.indiana.edu/~ucspubs/b131/">Gnu Emacs Reference Card</A>