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.