Touchpad taps have been disabled in Ubuntu Karmic and possibly some other distros based on Debian. Probably Debian as well because my research indicates this has made it into the current Karmic from some changes made to Xorg in Debian and brought into Ubuntu Karmic.
The developers in their great wisdom strike again. Now no offence developers but I hope you are reading all the posts on the Ubuntu Forums, a poll on Brainstorm and bug reports on Launchpad. This end user thinks the choice to disable touchpad taps by default was completely absurd. I hope it gets changed before Karmic is released.
The good news is it's very easy to change the behavior. Just click "System > Preferences > Mouse > Touchpad tab" and click the enable radio button. A screen shot is below...
See, very easy to restore what most folks consider normal reasonable behavior.