In the last several months (and two very recently) I've seen three really good examples people using Google Earth to explore new transportation ideas.  My favorite, both for its emphasis on transportation demand management strategies and because of its aesthetics, is Nick Falbo's exploration of Columbia River Crossing alternatives.

Columbia River Crossing : Alternatives from Nick Falbo on Vimeo.

Other videos using Google Earth worth checking out are Erik Halstead's proposal for a Portland-to-Tigard subway alignment (hat tip to the Tweeter @pdxcommute) and the North Portland Greenway's proposed trail alignment.  Cool!