This has happened to me many times, and just today it came to bite a friend of mine who was earnestly giving a try to Ubuntu...and renounced after losing his graphical interface except for bulletx. The problem is simple: sometimes kernel updates are released hours or even days before the restricted drivers packages are updated too (read: recompiled and packaged for new kernel). As a result, obviously, when the system is restarted the graphical driver does not function anymore.
And I am sure that it can happen also with other drivers.
The solution? One of two; either:
the kernel packages are completely held back until ANY AND ALL restricted drivers packages are updated (which is already done in Hardy, to be honest, considering that so far I have done three or four kernel updates and alwyas I saw in contemporary a new, repckaged nvidia-glx-new), or:
the update-manager program should check if any restricted drivers are installed, check if they are up-to-date with the kernel update, and in case they are not, blacklist the kernel updates until such a time when updating is secure.
I think this feature is quite a must; most users in the future are not even going to check what they are going to update at all; and it is sure to turn many people off Ubuntu if they suddenly find their boxes without graphical, or network (potentially even worse considering we have bulletproof X!) support.