Powered by the Unreal Engine, the is a fully 3D-rendered version of the old Bay Bridge we know and love. You'll start on the Oakland side in an overhead view that shows you exactly where the Eastbound detour is taking place; then, once you've gotten your bearings, you can drive alongside your fellow 3D-rendered passengers.
Once you successfully go through the detour once, you'll unlock the ability to drive on the original bridge in both directions (in case you're nostalgic for those pre-detour days); there are two future routes labeled "coming soon," as well--perhaps to show poor, beleaguered drivers what they have to look forward to as the bridge-building progresses.
The Bay Bridge Explorer app is a free download from the App Store for any iPhone 3GS or later, third-generation iPod touch or later, and iPad or iPad 2 running iOS 3.1.3 or later. From own tests on an original iPad, the app seems to be fairly high-maintenance and more than a little finicky; I'd recommend running as few other apps in the background as possible if you're planning on giving it a test drive.