An iPad 2 battery contains about 90 kilojoules of energy. This is equivalent to about 20g of TNT, which is only 1/3 of the TNT used in a WWII-era "pineapple" hand grenade.

However, the real reason one can bring a laptop battery on a plane is because no terrorist has tried to use it yet. They would probably need multiple batteries: after the "shoe bomber" attack, the government determined that as little as 50g of PETN could bring down a plane, which is equivalent to 80+g of TNT, which is four iPad 2 batteries.

Even then, it would be necessary to deliver that energy in a very small amount of time, which may not be possible rapidly enough with lithium-ion; they can burn, but can they explode quickly enough, with all of their energy? The TSA aays they have studied the issue, and the answer is no [1], they "cannot be used as an explosive and are not a security threat in personal carry-on quantities."

[1] http://usatoday30.usatoday.com/money/industries/travel/2010-...

Because you can ban water bottles, and sell them in the shop that's after security checks, and that won't turn many people back. But banning laptop batteries would harm business travel really seriously - it would really stop very, very many business travellers.