Single-display VGA output doesn't work after Gutsy upgrade

Following an upgrade to Ubuntu 7.10, my Dell Inspiron 1520 laptop is no longer capable of using only the external display, a Dell SE198WFP widescreen LCD connected by VGA to a NVIDIA GeForce 8400M GS. It worked well in 7.04 and even adapted the resolution to suit the active display, albeit with unofficial packages of xserver-1.3 and the NVIDIA proprietary driver. The following behaviour is observed:

The key combination for switching between outputs no longer works: display redrawing pauses briefly, but the external display doesn't do anything and the internal panel doesn't go blank. This happens whether or not the monitor's connected at startup.

If the external display's connected at startup, then the BIOS boot messages and usplash (at shutdown, too) appear on it: however, when X is started, it reverts back to using the internal panel. Virtual terminals are shown on the external display, but the text is visibly distorted and waves left and right. If the external display isn't connected at startup, the VTs are shown properly on the internal panel.

The displays can be set up with displayconfig-gtk to work in a dual head configuration, and this works well, but there seems to be no way to only use the external one. Marking the internal panel as "Disabled" and instructing it to use the external display as the default screen doesn't work at all: the internal panel tries to start using the resolution of the external display, fails several times, and ends up in a corrupted version of the X failsafe environment, with garbage displayed at the top and right hand sides of the screen and the rest of the display truncated to fit.

This worked well with an earlier version of the X server (1.3.0.0 from upstream) and of the NVIDIA drivers (100.14.11), so this could be a misconfiguration issue caused by the upgrade, but I've tried changing every relevant property I can find in xorg.conf with no luck. Any advice you may have would be appreciated!