http://www.bbc.co.uk/news/world-south-asia-13508864

Not sure who was behind it:

Here are some possibilities:

US: they have hundreds of undercover ops and special forces in pakistan. Further discredits pakistani security services

ISI: message to the pak army to keep hands off the ISI in light of upcoming purge as consequence of apparent 'bin laden' killing...links made to ISI and militants etc

India: revenge for mumbai..

Taliban...unlikely considering level of sophistication of the attack...