Symptom: System prints "Freezing Processes", then hangs forever.
Solution: add "hsf" to the RestartServices line in /etc/hibernate/hibernate.conf :
RestartServices hsf postfix alsasound
hsf is the Conexant HSF Softmodem.
The power button is not usable for suspend: it only triggers after a 3-second press, but after 5 seconds, it forces a poweroff. I can't see any way to make the button trigger after a light tap. So, plan B: use Fn+F12. To get acpid to recognize it, I followed the instructions at thinkwiki. The gist of it is:
echo enabled,0xffff > /proc/acpi/ibm/hotkeys
...and write a custom /etc/acpi/actions/ibm/hotkey script which checks the key and runs hibernate.