I've noticed more than once a very high cpu usage on OSX and it turns out that the culprit was the mdworker process from Spotlight (text indexing service).
There seems to be several causes to this. In my case the reason was that it was trying to index the contents of a zipped file currently being downloaded by Transmission (bittorrent client). The same could probably happen for any other zip file or directory full of files being downloaded by another tool.
The solution was to add my download directory to Spotlight's ignore list: System Preferences -> Spotlight -> Privacy tab -> Drag your typical download location to the list.
Restarting Finder and Spotlight after that is a good idea.