Comment 5 for bug 868684

Looks likely to be this.

gnome-settings-daemon (3.2.0-0ubuntu4) oneiric; urgency=low
  * debian/patches/00git_dont_sleep_on_idle_by_default.patch:
    - Don't sleep on idle by default (LP: #860485)
  * debian/patches/00git_dont_revert_pre_idle_brigthness.patch:
    - Do not revert to the pre-idle brightness if idle dimming is disabled
  * debian/patches/50_add_dell_backlight.patch:
    - Add 'dell_backlight' module to gsd-backlight-helper (LP: #862474)

That's the only thing that changed on the 3rd/4th that looks relevant. No X or kernel packages changed; there was one change to compiz but it looks innocuous. So, probably a regression g-s-d.

If you find the issue is still reproducible with current oneiric, you could maybe try downgrading to 3.2.0-0ubuntu3, and/or back out each of the changes to see which caused the vblank problem.