Hello, 

I don't have exactly same problem, but I found some problems with xorg when I switch user between openned sessions: 

PROBLEM: I have session with 1600x1050 and switch to another user's session that was previously set with same 1600x1050 resolution. But when I switch to this session, the screen gets 1280x1024 and for some reason the size of the session is very very wrong (it is bigger than the screen resolution and I can't move or click most of the things so I have to restart all Xorg). 

WORKAROUND: Fortunately I have found some way to resolv it in a tricky way: 
Change resolution file name with your resolution file. 

    1-Install get-edid command:  sudo apt-get install read-edid
    2-Create a bin file with edid information: sudo get-edid  > 1600x1050.bin
    3-Create a folder to store this file: sudo mkdir /lib/firmware/edid
    4-Copy the file: sudo cp 1600x1050.bin 
    4-Edit grub to edit linux command to load this file in kms: 
         -sudo editor /etc/default/grub
         -Edit line:  RUB_CMDLINE_LINUX_DEFAULT="drm_kms_helper.edid_firmware=edid/1600x1050.bin quiet" 
                 Change your name.bin file. 
         -sudo update-grub
     5-Reboot and test. 

For some reason it worked for me very well. 
I have this card and driver: 
fglrxinfo 
display: :0  screen: 0
OpenGL vendor string: Advanced Micro Devices, Inc.
OpenGL renderer string: AMD Radeon HD 6450
OpenGL version string: 4.2.12002 Compatibility Profile Context 9.012

My edid information file is: 

command: #parse-edid < 1600x1050.bin 


parse-edid: parse-edid version 2.0.0
parse-edid: EDID checksum passed.

        # EDID version 1 revision 3
Section "Monitor"
        # Block type: 2:0 3:fc
        Identifier "22T41-H-AN"
        VendorName "KTC"
        ModelName "22T41-H-AN"
        # Block type: 2:0 3:fc
        # Block type: 2:0 3:fd
        HorizSync 15-68
        VertRefresh 49-61
        # Max dot clock (video bandwidth) 150 MHz
        # DPMS capabilities: Active off:no  Suspend:no  Standby:no

        Mode    "1280x720"      # vfreq 60.000Hz, hfreq 45.000kHz
                DotClock        74.250000
                HTimings        1280 1390 1430 1650
                VTimings        720 725 730 750
                Flags   "+HSync" "+VSync"
        EndMode
        Mode    "1680x1050"     # vfreq 59.883Hz, hfreq 64.674kHz
                DotClock        119.000000
                HTimings        1680 1728 1760 1840
                VTimings        1050 1053 1059 1080
                Flags   "-HSync" "+VSync"
        EndMode
        # Block type: 2:0 3:fc
        # Block type: 2:0 3:fd
EndSection


kernel.log:
2139 Oct  2 21:45:06 dans679-computer kernel: [    0.000000] Kernel command line: BOOT_IMAGE=/boot/vmlinuz-3.8.0-30-generic root=UUID=939b3111-4906-     46ae-9ad7-7faa783d490d ro drm_kms_helper.edid_firmware=edid/1600x1050.bin quiet

I was looking for other bug report similar in launchpad, but still looking for the correct one. I would like find a user with same problem I had, because I think this problems affects many things in KMS (Kernel mode switch), not just switching between user session, also games in full screen with different resolution and others things. 

It is very hard to find the correct way to report this issue, I would like to have some help with some Xorg/Kernel team. 

I think this could be fixed in some low level, but could be also fixed with this kind of workaround in some easier way for users: For example adding some script that test if edid is detected or not, and asking the user if he liked to let script get the edid info and parse it to the kernel command, then ask if it was working fine and if the user wants to keep these settings. 

Hope it could help your issue too. 

Cheers,
Pablo.