This Launchpad bug discusses restoring the 'save session' functionality. Note: as described at the end, this official tweak does not appear to work - you may want to skip directly to B.Trying to enable the old Gnome session saving functionality Part B of this answer describes how to use an alternative hibernation method, TuxOnIce, which is more compatible and reliable right now than the built-in hibernation. The short answer is: hibernation is the best option because the gnome-session-save functionality was removed after 11.04 since it was buggy, and the official tweak to manually re-enable this functionality does not appear to work.
