You may run into this issue after upgrading to Lion or after doing some system maintenance that deletes the Help system files (same thing with Spotlight which is managed by mdworker). If that happens, just let it run until it's finished.
HIUTIL and MDWORKER CPU. Immediately after installing Lion, and over a period of a few hours, the hdutil and mdworker processes will be using up 90% to 100% of your CPU making the computer very slow. These tasks are used to update the Spotlight index and help file index. So, it’s best to wait until these programs have completed their tasks before continuing to work on the computer.