Finally I succeeded to resolve my very irritating issue of getting my mac to stay in sleep mode. Every time sleep was requested, either from the set idle timeout or manually, the mac instantly woke up and left a "Wake event 0020" mark in syslog.
Some posts on the net suggested this type of errors was related to usb keyboards but my mac didn't stay in sleep mode even if the keyboard was disconnected. After some unplugging of different peripherals I found out that my sleep issues was due to my usb hub/memory card reader from Hama. This usb hub I bought because we have similar devices at work and they work nicely (with windows) and are cheap. However, in a mac setup these sends all kinds of spurious stuff when the mac is entering sleep mode causing the mac to immediately go back in active mode again.
So, don't buy one of these if you have a mac.