I have Debian 6 (squeeze), I have also seen this under Ubuntu (can not remember how I fixed it).

I can hibernate, but when I switch on the system cold boots (it does not restore previous session).

Note suspend works fine.
Have looked in /var/log/pm-suspend.log Shows for each suspend suspend block a resume suspend block, but hibernate hibernate' is not followed by resume hibernate` ( I assume that this is not correct. )

Update on what I have discovered:

I have discovered that the uswsusp package is missing from debian squeeze. I could get it from testing or unstable repository, but have to be careful about not pulling in other dependencies. I would like to know if it will fix it. Is uswsusp needed for hibernate. Or should pm-utils be enough. (suspend it ram is working fine.)

This issue has now fixed it self, there is nothing in synaptics history to say that I installed something to fix it. It may have been an update, are these in the same history log? BUT Redetection of my 'option' modem is now slower.