Can't extend desktop onto second monitor, Radeon Mobility X700, Acer Aspire 3023

When attempting to set up a second monitor on my notebook, I can get the second monitor to mirror the notebook monitor but can't extend my desktop onto the second monitor. I'm running Ubuntu Gutsy Beta with GL desktop enabled. I haven't hacked xorg.conf at all.

If I run displayconfig-gtk from the command line, it fails with the following output when testing "Extend default screen..."

mdoube@doris:~$ sudo displayconfig-gtk
[sudo] password for mdoube:
Xlib: extension "XFree86-VidModeExtension" missing on display ":1.0".
on_button_test_config_clicked()
xauth: creating new authority file /tmp/dcg-aQfLig6175/xauthority
Got pid 6514
(0, 0)
displayconfig-gtk: Fatal IO error 104 (Connection reset by peer) on X server :9.0.

lspci -vv:
01:00.0 VGA compatible controller: ATI Technologies Inc Radeon Mobility X700 (PCIE) (prog-if 00 [VGA])
        Subsystem: Acer Incorporated [ALI] Aspire 5024WLMi
        Control: I/O+ Mem+ BusMaster+ SpecCycle- MemWINV- VGASnoop- ParErr- Stepping- SERR- FastB2B-
        Status: Cap+ 66MHz- UDF- FastB2B- ParErr- DEVSEL=fast >TAbort- <TAbort- <MAbort- >SERR- <PERR-
        Latency: 0, Cache Line Size: 32 bytes
        Interrupt: pin A routed to IRQ 23
        Region 0: Memory at c8000000 (32-bit, prefetchable) [size=128M]
        Region 1: I/O ports at 9000 [size=256]
        Region 2: Memory at c0100000 (32-bit, non-prefetchable) [size=64K]
        [virtual] Expansion ROM at c0120000 [disabled] [size=128K]
        Capabilities: <access denied>