I've upgraded virtualbox from 1.6.0 to 1.6.2 (including guest-additions) and now I'm stuck at 1024x768 on my linux guest (on a 1920x1200 monitor!). I have "Auto resize guest display" turned on and on 1.6.0 it worked fine: I could used xrandr or start the X server with the correct resolution.
I'm using OS-X 10.4 on a Macbook pro. My guest is a Mandriva Linux 2008.1. Virtualbox 1.6.2 (including guest additions), installed from virtualbox.org.
I followed the procedures from tickets #1689 and #1691 (apparently related to this bug), but they didn't solve my problem.
I also tried to debug the problem with someone from IRC (who reviewed all the guest-additions installation, my vm configuration and the VBoxManage commands), without success. :-(
Again, it was working without any problem with 1.6.0 (I use the guest daily, it's my desktop and development platform).