I reported this to Ubuntu a while ago, and I guess it hasn't gone anywhere.
https://bugs.launchpad.net/ubuntu/+source/xf86dga/+bug/88585

 I was running a multiseat setup on the machine, but I've reproduced the problem on Ubuntu Feisty (xorg 7.2) with a single-head g965 setup.  Maybe it's an amd64 problem, since multiple drivers are affected.  Thus, I've marked this as being a "server/general" problem.  I hope that's close.

 Next time I'm going to reboot anyway, I'll try to remember to give DGA a try with the X server from my Ubuntu Gutsy chroot (xorg 1.3.0).

--quoting my first report--
 I run AMD64 Ubuntu Edgy on a core2duo with onboard g965 and PCI r128 video hardware, in a multiseat setup. (-sharevts, evdev input devices).

Running sudo dga makes my screen blink (i.e. a relay in the CRT clicks, so the video signal must have been affected), and the screen stops updating. The X server also stops reading the keyboard.

  The X server was still running, and xdpyinfo worked. Killing my wm made my startx session exit, including the X server. Starting X again didn't show any signs of video corruption, so it didn't leave the card in a weird state. (the text console was screwed up, of course.)

 This happens on both the i965 head and the r128 head. crashing one doesn't affect the other.

 The interesting part is that the kernel log shows that the dga process had a corrupt pagetable. I'll attach my kern.log, from doing dga on the r128 head, and then on the i965 head.
---

 I'm not going to copy over all the attached files.  One snippet of the kernel log is from running "dga" on the r128 head:
Feb 27 22:27:28 tesla kernel: [199889.915094] dga: Corrupted page table at address 2b4951974000
Feb 27 22:27:28 tesla kernel: [199889.915131] PGD 83c7067 PUD 3a546067 PMD 179d8067 PTE ffffffff90000027
Feb 27 22:27:28 tesla kernel: [199889.915169] Bad pagetable: 000f [1] SMP 
Feb 27 22:27:28 tesla kernel: [199889.915199] CPU 1 
Feb 27 22:27:28 tesla kernel: [199889.915222] Modules linked in: ipx p8023 nfs binfmt_misc rfcomm l2cap bluetooth nfsd exportfs lockd sunrpc ipv6 autofs4 cpufreq_userspace cpufreq_stats cpufreq_powersave cpufreq_ondemand cpufreq_conservative video sbs i2c_ec dock container button battery asus_acpi backlight ac ext3 jbd mbcache xfs fuse r128 i915 drm eeprom i2c_i801 i2c_core acpi_cpufreq freq_table sbp2 lp sr_mod cdrom ata_generic tsdev joydev sg pata_marvell iTCO_wdt iTCO_vendor_support intel_agp af_packet e100 mii parport_pc parport psmouse serio_raw e1000 pcspkr evdev snd_hda_intel snd_hda_codec snd_pcm_oss snd_mixer_oss snd_pcm snd_timer shpchp pci_hotplug snd soundcore snd_page_alloc jfs usbhid hid raid456 xor raid10 raid1 raid0 linear md_mod dm_mod ehci_hcd ohci1394 ieee1394 uhci_hcd usbcore ide_generic generic sd_mod ahci libata scsi_mod thermal processor fan fbcon tileblit font bitblit softcursor
Feb 27 22:27:28 tesla kernel: [199889.915668] Pid: 3260, comm: dga Not tainted 2.6.20-6-generic #2
Feb 27 22:27:28 tesla kernel: [199889.915702] RIP: 0033:[phys_startup_64+1359450960/2147483392]  [phys_startup_64+1359450960/2147483392]
Feb 27 22:27:28 tesla kernel: [199889.915742] RSP: 002b:00007fff5a2b0988  EFLAGS: 00010202
Feb 27 22:27:28 tesla kernel: [199889.915775] RAX: 0000000000040000 RBX: 0000000000000001 RCX: 00002b4951974000
Feb 27 22:27:28 tesla kernel: [199889.915830] RDX: 0000000001000000 RSI: 000000000000000d RDI: 00002b4951974000
Feb 27 22:27:28 tesla kernel: [199889.915884] RBP: 0000000000502010 R08: 0d0d0d0d0d0d0d0d R09: 0000000000000001
Feb 27 22:27:28 tesla kernel: [199889.915938] R10: 0000000000000000 R11: 00002b4951279390 R12: 0000000000000000
Feb 27 22:27:28 tesla kernel: [199889.915992] R13: 00007fff5a2b0c00 R14: 00007fff5a2b09d0 R15: 0000000000000000
Feb 27 22:27:28 tesla kernel: [199889.916047] FS:  00002b4951973390(0000) GS:ffff81007f682dc0(0000) knlGS:0000000000000000
Feb 27 22:27:28 tesla kernel: [199889.916104] CS:  0010 DS: 0000 ES: 0000 CR0: 000000008005003b
Feb 27 22:27:28 tesla kernel: [199889.916137] CR2: 00002b4951974000 CR3: 000000002800a000 CR4: 00000000000006e0
Feb 27 22:27:28 tesla kernel: [199889.916192] Process dga (pid: 3260, threadinfo ffff81003edda000, task ffff8100199cb820)
Feb 27 22:27:28 tesla kernel: [199889.916248] 
Feb 27 22:27:28 tesla kernel: [199889.916270] RIP  [phys_startup_64+1359450960/2147483392]
Feb 27 22:27:28 tesla kernel: [199889.916299]  RSP <00007fff5a2b0988>
Feb 27 22:40:43 tesla kernel: [199889.916479]  <3>[drm:drm_lock_take] *ERROR* 3 holds heavyweight lock

... and now the g965 head:
Feb 27 22:50:00 tesla kernel: [201241.508192] dga: Corrupted page table at address 2b685b784000
Feb 27 22:50:00 tesla kernel: [201241.508229] PGD 64856067 PUD 2c343067 PMD 2ef46067 PTE ffffffff80020027