http://techon.nikkeibp.co.jp/english/NEWS_EN/20071026/141410/ The article suggests that Toyota are looking at 420 kg (926 lbs) as being the minimum mass for this shape of vehicle (for stability / centre of mass / handling issues with four occupants). Apparently this car still has the same internal volume as the Prius. Presumably some large auto company will have to go through the very expensive process of re-writing the crash test standards for these new spaceframes, before low-volume manufacturers can start to make them. Final mass is likely to be significantly higher in response to testing and once all the usual gadgets are added. Just as well really. As cars become lighter the distribution of passengers and luggage starts to play a greater role (roll?) in stability.