"""Zardari enjoys immunity from prosecution as a head of state, Prosecutor-General Daniel Zappelli told Reuters....
....“In Pakistan they decided that no crime had been committed,” he said.....
.....Zappelli also noted that Zardari and Bhutto had been sentenced by the High Court in Lahore in 1999, but in 2001 Pakistan's Supreme Court had cancelled this verdict and sent it back to Lahore for a new decision. However, there had not been a new trial in the nine years since then.....
....In any case under international law Zardari enjoyed immunity from prosecution as a head of state - unless that state itself lifts the immunity......
“Immunity is the key question,” Zappelli said. “We can't prosecute Mr Zardari while he has immunity unless Pakistan lifts that immunity. And if he doesn't have immunity, why don't they try him in Pakistan?”
http://www.bbc.co.uk/urdu/pakistan/2010/03/100331_sc_nab_response.shtml