Fixed with this upload:
gdm (2.16.1-0ubuntu3) edgy; urgency=low
.
* debian/patches/11_powermanagement.patch:
- updated to make hibernate work from the login screen too (Ubuntu: #3642)
** Changed in: gdm (Ubuntu)
Status: Confirmed => Fix Released
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hibernate doesn't work from gdm itself
https://launchpad.net/bugs/3642