There are two things at work here, as far as I can tell:
While the Dell is configured in the "Monitor" section, it is apparently 
configured as a VGA output:

from line 477
(II) intel(0): Output VGA using monitor section DELL2407WFPH

That monitor section came directly from a dpkg-reconfigure xserver-xorg.

Xorg does appear to automatically detect the TMDS display. It goes on to
say that VGA is disconnected, and it shows resolutions for LVDS and TMDS

lines 750-754
(II) intel(0): Output VGA disconnected
(II) intel(0): Output LVDS connected
(II) intel(0): Output TMDS-1 connected
(II) intel(0): Output LVDS using initial mode 1440x900
(II) intel(0): Output TMDS-1 using initial mode 1280x1024

The last line is where we have the problem: just above it lists every other 
video mode with
[lines 684-735]
(bad mode clock/interlace/doublescan)

So apparently Xorg isn't properly detecting the modes correctly.
Unfortunately, I don't have another monitor to test with.

That said, the monitor does support 1280x1024, so it's odd that it still
gives no signal until after I've logged in. I am able to correctly
reconfigure the screens using xrandr, though gdm still won't work.

The Dell 2407-HC supports these modes:
Display Mode    H (kHz)         V (Hz)  Pixel Clock (MHz)       Sync Polarity 
(Horizontal/Vertical)
VGA, 720 x 400  31.5    70.1    28.3    -/+
VGA, 640 x 480  31.5    59.9    25.2    -/-
VESA, 640 x 480         37.5    75.0    31.5    -/-
VESA, 800 x 600         37.9    60.3    40.0    +/+
VESA, 800 x 600         46.9    75.0    49.5    +/+
VESA, 1024 x 768        48.4    60.0    65.0    -/-
VESA, 1024 x 768        60.0    75.0    78.8    +/+
VESA, 1152 x 864        67.5    75.0    108.0   +/+
VESA, 1280 x 1024       64.0    60.0    108.0   +/+
VESA, 1280 x 1024       80.0    75.0    135.0   +/+
VESA, 1600 x 1200       75.0    60.0    162.0   +/+
VESA, 1920 x 1200       74.0    60.0    154.0   +/-
according to the specifications.

** Attachment added: "xorg log from gdm startup"
   http://launchpadlibrarian.net/9888022/Xorg.0.log

-- 
gdm refuses to display on external monitor
https://bugs.launchpad.net/bugs/148189
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