i know that this problem is well known, anyway, i still seek an answer for it:

taking the latest 3 drivers from ati:
8.14.13
8.16.20
8.18

8.14.13 works quite well, but it lacks GCC4 support - a bit outdated.
8.16.20 works crippled (see below)
8.18 works, but the only reason i need 3D-acceleration (playing wow) doesnt work - the error is rather strange:
i can start wow (with newest cedega), i can login, i come to the selection screen (where everything is displayed correctly - so 3D itself is working), but when i select a character, it starts to load (the screen with the progress-bar comes), loads to 100% and then - stays forever there...

now for the 8.16.20 resolution problem (which is the reason i post):
i have a laptop with radeon9000m and a 1400x1050 screen
the 8.16.20 drivers work quite well, but they just work with 1280x1024 (maximum, of course 1024x768, 800x600 and so on work...) - and NOT with 1400x1050.
the xorg.conf itself is ok, as its the same as worked with the 8.14.13 driver.
the driver gets loaded and is used (glxinfo says so, glxgears gives good values - even more than the 8.14.13 driver, cedega+wow works).

but only at 1280x1024

anyone has an idea how i can workaround that bug in the driver?

No, but I will be following your progress closely in hopes it may lead to an answer of my issue with no 3d acceleration on my ATI. I will keep looking as well, and let you know if I find out anything.