I'm having the same issue right now with fully updated Zen, though I successfully as root ran pm-hibernate and it worked fine twice. Incorporating this into a script (for mythtv) same issue happened here - black screen but I have a blinking cursor top left - but system completely unresponsive (have not tried sysctrl-reisub reset though),
testing pm-hibernate and pm-suspend some more tomorrow
EDIT: after a hard reset system is starting normally