Spotlight constantly indexes MacBook Air (Lion). Takes up 50+% of cpu
Try to erase the Spotlight index by either running sudo mdutil -Ea or by temporarily adding your startup disk to the privacy tab in the Spotlight preference pane.

If that doesn't work, try to run:

sudo lsof|grep mdworker


or 

sudo mdutil -Ea;opensnoop -n mdworker


to see if the indexing hangs on some specific files.