Linux - How to get the reason for wakeup?

Is it possible to get the reason of system wake-up (Wake-on-LAN, Power button, wake from USB) on Linux? I'm using Ubuntu, kernel Linux homeless 2.6.37-12-generic #26~lucid1-Ubuntu SMP Wed Jan 5 21:52:06 UTC 2011 i686 GNU/Linux

Depending on your hardware, you can find this using dmidecode:

$ sudo dmidecode |grep Wake-up
    Wake-up Type: Power Switch

There is lots of other useful hardware info from dmidecode, like the make/model/serial number of your motherboard/DIMMs/CPUs

In ubuntu it comes with the package dmidecode, should be available even in Lucid.

This sorta explains it at the hardware level. What do you see when you dmesg | grep -i 'acpi'?