Laptop monitor won't go 16:9 using 3.8.3 kernel
 

Using 64-bit -current, a rather odd problem. This didn't happen with 3.8.1, and is solved by reverting to 3.7.10 as well.

Everything seems to be normal during boot, but when I log in (I'm using E17 and slim) the monitor goes 4:3. I tried KDE, and it also starts in 4:3 although once everything is loaded up it abruptly switches to 16:9.

The hardware is:
00:02.0 VGA compatible controller: Intel Corporation Mobile 4 Series Chipset Integrated Graphics Controller (rev 07)
00:02.1 Display controller: Intel Corporation Mobile 4 Series Chipset Integrated Graphics Controller (rev 07)

But if that was the problem I don't think it would solve itself in KDE. I also don't think it's an X problem, as the console is also 4:3, albeit with the left side of the monitor used instead of the centre.

i remember seeing some i915 issues related to screen resolution reported for 3.8.3 on the LKML. So you'd best go back to 3.8.1 and wait for 3.8.4. Or revert the offending commit...