Testing the new branch called 'restart' of the hw-agnostic xf86-video-modesetting driver utilizing only the DRM/KMS API (and therefore also not providing the mode-set functions tuple .prepare/.fixup/.mode_set/.commit but only .set_mode_major), I found that xfce4-settings-manager's display configuration tool does not work reliably.

The branch can be found at git://anongit.freedesktop.org/xorg/driver/xf86-video-modesetting

What happens:

Clicking on a mode [B] from the list, bigger than the now active one [A], results an attempt to set the new mode, in an enlarged framebuffer and desktop (as can be seen by the panel disappearing as it moves out of the displayed area) but the real display mode staying the same at [A], as is reflected by the then active entry in the drop-down box (which appearently has changed back in this case).
Also the message "Failed to configure LVDS-0" appears in the log.
I've been printing out the mode line the xf86-video-modesetting driver gets in it's set_mode_major function, which in the above case is [A] instead of [B].

A second attempt to set the mode [A] that just failed works and now the setting also appears highlighted in the drop-down box.
Now there is no message in the log and the driver does indeed receive the selected mode [B].

Additional info:
The tools xrandr and Gnome2's gnome-display-properties work fine with xf86-video-modesetting.
I've tested 4.8.2 as packaged by Gentoo and also the latest git commit 62f287570ca0ffd44ce691273c4cdf4a387ded37 of xfce4-settings as of today. Same behaviour.

I've tested this with kernel 3.0.4 and 3.1.0-rc8+ (git, also as of today) and libdrm-2.4.26. I'm pretty sure these work as expected as xrandr and the gnome tool don't have any problems with any of these versions.

Please let me know if any additional information is required.