From the BBC news
When I worked for a major computer manufacture there were regular occurrences of laptop batteries overheating and 'bursting' on planes but none that I am aware of ever caught fire or exploded.
I also witnessed a potentially serious accident when a colleague was idly 'fiddling' with a two-way radio when the charging contacts on the bottom accidentally came in contact with a gold filigree chain he was wearing round his neck. The part between the contacts immediately melted and the chain round his neck started to rapidly heat up. Fortunately this made him drop the radio before any injury occured.
A direct short on a NiCad, NiMh or Li-on battery delivers a surprisingly high current.