Unfortunately such batteries have a greater likelihood to catch on fire or even explode ("spontaneously combust"). As sib blog engadget reports, it's one thing for a laptop battery to blow up; cars performing such pyrotechnics would have a larger impact (and possible lawsuits).
Enter Valence Technology. The Austin-based company has modified its U-Charge Power System lithium ion batteries to lower the chances for fires. Instead of using a cobalt oxide cathode found in most such batteries, Valence's batteries use metal phosphate which doesn't have the same tendency to burn when the battery gets hot. Unfortunately, such batteries only hold 75% of a charge compared to a regular battery. Engadget rightly points out that many drivers, especially in the U.S., may not be willing to lose efficiency in the name of safety.
[Source: CNET News via Engadget]