Lion doesn’t come with the same one finger click-and-drag gesture that used to be in Snow Leopard, and instead the user has an option of enabling a three finger drag gesture through the system preferences. This post shows you how to re-enable the one finger drag gesture.
To enable one finger drag, go to System Preferences, click on Universal Access, and then the Mouse & Trackpad tab. Now click on the “Trackpad Options…” button available at the bottom of this screen. Enable the second option, which is named “Dragging” and select the desired dragging behaviour from the drop down list: