I don't agree with this concept that GM can't bring out the Volt and Toyota is holding back. It is possible today--yes today---you can have a vendor install a plug in hybrid battery pack for the Prius and Escape models at Hybrid-plus location in Boulder. What is holding things up is documented testing/research leading to warranty and of course price. Once the manufactures document the product meets stated standards--a legal necessity--a warranty will follow. Mass production will drop the price tag. Of course so can tax credits. There is no
politician worth their re-election that will not support tax credits for this venture to make it cost effective for the Jane and Joe to purchase such a setup. Remember we are talking about today's vehicles fitted with the newest hybrid battery technology that can deliever over 100 mpg!
The former GM EV1 used a special high voltage charger where as today's plug in vehicles just need standard household outlet. A big difference. This country already has the infrastructure to support this move.

2005 AWD Escape Hybrid
Best Interstate
tank trip MPG 39.02 (scangauge II) for 402 miles on I-70, 10.3 gallons used over mostly flat terrain. Aug. 2007
Best
Interstate tank trip E30 MPG 34.6 for 271 miles along I-80 in Indiana and Ohio. May 2008
Best multiple road tank trip E30 MPG 36.2 for 202 miles in Southwestern Pa. July 2008.