I've noticed that from time to time, the tapping functionality for my glaspad just disappears. When I go into system preferences, it's still checked, when I uncheck and recheck it, it works again. Very strange. Is this a known bug?
Have you applied the firmware update released a few months ago? There was a firmware update for the trackpads which specifically addressed this issue. Should be able to get by running software updates or from Apple's support downloads page.