problem running graphical GNU Emacs 24

[Tom Roche]

summary: I can start emacs 24 in a terminal, but not graphically.

details: I'm running

$ lsb_release -ds
Linux Mint Debian Edition # Update Pack 3
$ uname -rv
2.6.39-2-amd64 #1 SMP Tue Jul 5 02:51:22 UTC 2011

I can currently run

$ emacs --version
GNU Emacs 23.3.1

both graphically (in an X window) and in a shell (terminal window), and want to 
be able to run emacs 24 both ways.

I backed up my sources.list, appended the debian emacs-snapshot repositories, 
added their keys, and updated:

$ diff /etc/apt/sources.list_20111122_0736 /etc/apt/sources.list
15a16,18
> # for emacs 24 stable: see http://emacs.naquadah.org/
> deb http://emacs.naquadah.org/ stable/
> deb-src http://emacs.naquadah.org/ stable/
$ wget -q -O - http://emacs.naquadah.org/key.gpg | sudo apt-key add -
OK
$ sudo aptitude update ; sudo aptitude -s full-upgrade
...

I then installed emacs for use in X and shell:

$ sudo aptitude install emacs-snapshot-el emacs-snapshot-gtk emacs-snapshot-nox
...
$ which emacs
/usr/bin/emacs
$ ls -alh /usr/bin/emacs*
lrwxrwxrwx 1 root root  23 Aug 14 22:53 /usr/bin/emacs -> 
/etc/alternatives/emacs
lrwxrwxrwx 1 root root   9 Aug 14 22:53 /usr/bin/emacs23 -> emacs23-x
-rwxr-xr-x 1 root root 11M Apr 10  2011 /usr/bin/emacs23-x
lrwxrwxrwx 1 root root  29 Aug 14 22:53 /usr/bin/emacsclient -> 
/etc/alternatives/emacsclient
-rwxr-xr-x 1 root root 21K Apr 10  2011 /usr/bin/emacsclient.emacs23
-rwxr-xr-x 1 root root 26K Nov 21 09:53 /usr/bin/emacsclient.emacs-snapshot
lrwxrwxrwx 1 root root  32 Nov 22 08:10 /usr/bin/emacs-snapshot -> 
/etc/alternatives/emacs-snapshot
-rwxr-xr-x 1 root root 12M Nov 21 09:53 /usr/bin/emacs-snapshot-nox

When I run

$ emacs --debug-init &

I get the graphical/X version of emacs, in a new window. But when I run

$ emacs-snapshot --debug-init &

I get

> [1]+  Stopped                 emacs-snapshot --debug-init

which, after `fg 1`, runs emacs 24 in the terminal. I expected, instead, the 
same behavior as emacs 23.

What must I do to start a graphical instance of emacs 24?

TIA, Tom Roche <Tom_Roche@pobox.com>