Some users have reported an issue in which MacBook and MacBook Pro trackpads, and the Magic Trackpad, appear to become stuck in “clicked” mode. Users experiencing this problem may accidentally highlight text, drag items or perform other activities.
This issue may occur after the trackpad software update or the Mac OS X 10.6.4 update.
As a temporary workaround (to eliminate the stuck click), you can open System Preferences then select Trackpad and deselect “One Finger – Tap to Click” then reselect “One Finger – Tap to Click”
Some users have been able to resolve this issue by reinstalling Mac OS X.