Lion Mail: Drag to Select vs. Drag to Move
Selecting happens when you start dragging in the empty part of a line and have a major vertical movement. You can prevent message moving from happening by immediately starting to drag across after clicking.

Moving happens when you start in the text part of a line. Moving is also easier invoked when your major movement is in the horizontal axis. If you click and hold on a spot for a moment and then start dragging message moving will be invoked.