I had this issue with Idea 4.5 as well as I do with Idea 5.
When Idea is not active, window is not active, it uses around 20% CPU time all the time. When I switch to it - nothing changes, same usage. "Compiling in background" is not checked. And this CPU load doesn't go away until I restart Idea. And I'm not in the debug session or anything at that time.
This is not 100% repeatable, but happens quite often. Goes away when I restart Idea.
Environment: Win2K Server, P4 Xeon, hyperthreaded