My two monitors run on different resolutions (laptop 1440x900, HDMI 1920x1080). I set them up as an extended desktop and they work just fine
However, while logging in, the greeter appears to be set to "mirror displays" and seems to be running at a lower resolution on both screens. This does not only look bad but makes my HDMI monitor turn itself of and on again (it does that with unsupported resolutions)
The problem appeared after I did a fresh install of oneiric, it worked just fine on Natty. The laptop is a Dell Inspiron 1525 that came with Ubuntu and it has an Intel graphics chip:
00:02.0 VGA compatible controller: Intel Corporation Mobile GM965/GL960 Integrated Graphics Controller (primary) (rev 0c)