The Intel i855 (along with the i830 and i845) chips have suffered serious stability regressions since the implementation of DRI2 (and the associated GEM kernel work) in the driver. These features have stressed parts of the hardware that haven't previously been heavily used in the past, and turn out to have hardware bugs.
Thus, in Ubuntu 10.04 we turned off KMS for these cards. That didn't help enough, and many users still reported frequent system crashes - from X just not coming up, to the system hanging whenever they tried to play a video, to apparently random freezes every couple of days.
In Maverick we've just disabled the autoloader for the Intel driver on these cards. That should give a baseline stable experience for users of these cards, dropping back to
See this page for details of how to use the Intel driver (and possibly sacrifice stability).
Update: The Intel driver will be autoloaded in the upcoming Ubuntu 11.10 release, as upstream has (apparently!) worked around the issues that this hardware has.