The latest news from the DeWalt power tool nano tech lithium ion battery release is that they promise 2 to 3 times the operating time over the 18 volt model.  The 18 volt model has a 2.4 amp hour battery.

So around 7 amp hours are stored in the 36 volt lithium ion battery?  If so each battery holds around 250wh.  Since each battery weighs 2.4 pounds, 7.5 kwh of storage, the equivalent energy to one gallon of gasoline, would weigh about 70 pounds.

It looks like the NIT/A123  lithium ion nano batteries may weigh in at around 70 pounds per gallon of gasoline  energy equivalent. They are now being used in a car project by Altairno.  And a similar Hitachi battery design is being used in a Subaru test vehicle.

That's 350 pounds for a 5 gallon range.  Say 200 miles in an economy car?  With the rest of the electric components at around 150 pounds, that is 500.

Remove the motor, transmission, and related items from an economy car.  Would they weigh 500 pounds?

Pretty close.  Maybe the electric version could save weight with lighter versions of easily replaced body components.  Save 100 pounds that way and the electric version will weigh and perform the same.  With a range of 200 miles.

Add another 70 pounds of batteries for a 240 mile range?  It would depend on driving habits, extension batteries could be added for longer trips.  The added weight would be well within the vehicle's cargo capacity