Comment 8 for bug 40883

This bug is a duplicate of 199006
The default status of PS/2 and USB keyboards in /proc/acpi/wakeup is set to 'disabled'.

This can be easily changed for example by running (as root):
echo "PS2K" > /proc/acpi/wakeup

Attached is a script that can be used to enable this support by default. I use it under /etc/acpi/suspend.d/

For a complete discussion see:
http://ubuntuforums.org/showthread.php?t=711747