There was one app that could do it, Coolbook. I'm not sure if it's been updated for Lion or Mountain Lion. Another one might be CPU-X. It's a port of CPU-Z, but it hasn't been updated since 2007/2008. If you can find a copy of it, you could try it as well.
PS: CPU's in Mac OS X systems can and do run with 0% usage on at least one core. If there's only two active threads and four logical cores, two will be at zero percent. It's rare, but it does happen.