For some (sightly odd, imho) reason, Ubuntu decided to disable hibernation by default in the latest release - 12.04

Fortunately re-enabling hibernation (on machines that support it) is reasonably simple, with step by step instructions on help.ubuntu.com and askubuntu.com.

This is one of those occasions where I believe Ubuntu made the wrong decision, it might save them bug reports from unsupported hardware, but disabling a useful - working - feature on many machines is never a great idea.