Benazir Bhutto's self-imposed exile came about because she was facing hundreds of corruption charges. Although the charges may have been brought against her for political reasons, its likely that the majority of them were well-founded.
Pervez Musharraf 'won' the recent election, in the sense that Pakistan's electoral commission confirmed that he received the majority of the votes. However, his eligibility to serve as elected president is still subject to a legal ruling from Pakistan's courts. (The courts must decide whether the constitution simply requires him to relinquish his military post, in which case he can serve, or whether his nomination and subsequent election were void because he was in the army at the time of his nomination and election).
The uncertainty over the election results has weakened Musharraf's power in Pakistan. The USA, UK and other western countries want him to stay in power because of his anti-extremist stance. He has been put under pressure to enter into an alliance to strengthen his power base. Accordingly, he has dropped all of the charges against Benazir Bhutto, so that she can return to Pakistan and negotiate a possible alliance which might see Benazir Bhutto become Prime Minister again. (However, that can only happen with constitutional changes).
Benazir Bhutto has already expressed her support for American forces being given the right to operate within Pakistan against extremist forces. Unsurprisingly, this has made her the target of threats and attacks from those same extremists.
Chris