Actually - they key change is that to drag a control around in VS2008, you drag the white tab ABOVE the control, not the control itself. If you drag the control of the window, you're changing the parent.
http://blogs.msdn.com/mikhailarkhipov/archive/2008/03/05/absolute-and-relative-positioning-in-visual-studio-2008-web-designer.aspx
This is a stubtle change to the way it worked in VS2005.
But the reminder about using the ALT key to turn off grid/snapping as well is useful as well.
I googled so many times to try and find this!
Cheers, Rob.