Hi guys, just wanted to chime in and say that I'm also seeing super high CPU usage. My fans kick on high, and when I check activity monitor I see the same thing as ZeoS: near-100% CPU usage. It's not constant, it just comes in spikes.
Today is the first day I've ever used Avast on the Mac (I've used the Windows version for years), so please let me know if I can share any information that'd help track down this annoying problem.
I'm running the latest version of Lion w/ FileVault turned on.
Preliminary debugging shows that the spikes go away when File System Shield is disabled in preferences, but it's only been about 10 minutes since I did that so I can't be sure.