So, this problem appears common on other phones as well, and appears to be a Gingerbread issue. Drilling down further using Watchdog Lite on the phone, it appears to be a process called Suspend causing the high CPU usage.
There are a few threads on this:
Personally, I'm getting pretty terrible battery life out of my S2, and I can confirm that in the Battery Status menu, Android OS appears extremely high at around 40% (a friends Nexus has this sitting at around 2%).
Not sure there's much we can do about it but wait and hope for a fix from Samsung (or Google)... People are reporting that rebooting the phone can help, but it's only temporary.
Anyway, just something to check if you're having issues.