Pedro Ferreira sent this tip. "When I want to create a 3D solid of an area in a drawing, for example, a wall in a house, I use Ctrl + Alt and pick the area where I want to create the 3D solid. This area can be defined by lines, lwpolylines, regions, and more, and if I move the cursor while pressing Ctrl + Alt, it shows where I can create this solid. This tip was tested only in AutoCAD 2008 full version (English) with SP1, and I think the Ctrl+Alt keys are undocumented."

NOTES FROM CADALYST TIP PATROL: True, it's undocumented, but it works just as Pedro says. A 3D solid is created on top of the objects used to make it; these objects are not erased. It also works in 2009 versions of AutoCAD MEP and Civil 3D.