I tried my Oct 6th 2011 test case with Emacs 24.0.91.  24.0.91 is
definitely much better.  Emacs no longer hangs when I type "quit" at
the (gdb) prompt; instead Emacs offers the following in the
minibuffer:

   Buffer "*input/output of foo*" has a running process; kill it? (yes
   or no)

That seems much nicer than the behavior under 24.0.90.

I also tried the `exit 0' case I provided earlier (using a C-source
file called foo.c):

   - M-x gdb RET
   - press RET at the minibuffer's "Run gdb" message
   - type "set args x RET" at the (gdb) prompt
   - type "run RET" at (gdb) prompt

Performing these steps causes (the compiled) foo.c to run and exit(0).
Aferwards, I see "Debugger:run [exited-normally]" in the mode line.
The text `exited-normally' is colored red -- should it be green for
normal termination of the debugged program?

Red vs. green is really a minor thing.  I believe this issue can be
closed.

Steve