A Russian software company, LEDAS of Novosibirsk, has figured out how to make SketchUp do parametric editing.
Driving Dimensions plug-in gives Google SketchUp users a unique combination of wide capabilities with an easy-to-use tool for parametric editing of 3D models.
At this point, the plug-in is limited to driving two types of dimensions: linear (changing the lengths of lines) and radial (changing the radius of circles and arcs).
Future releases are planned for adding distances, angles, geometric constraints (parallelism and perpendicularity, among others), and dynamic reconciliation of constraints of moved model elements.
And the company plans to make their Driving Dimensions work with other CAD software that lacks parametrics -- AutoCAD being the obvious next target, I figure.
Works with the free and Pro versions of SketchUp 6 on Windows. Download for free from drivingdimensions.com