According to the BBC, an apparent terrorist attack killed
supporters of returning Pakistani ex-PM Benazir Bhutto while she was touring the country as part of her campaign for her party, the PPP, to win the parliamentary elections and to regain the position she lost due to corruption charges. The news now is sketchy, but the attack was apparently close to her convey but it is believed she is safe. Bhutto just returned to the country today.
The BBC reports that body parts were strewn across her truck as it sped away. The Telegraph reports that the bombs appear to have been detonated by suicide bombers just feet away from Ms. Bhutto’s truck, shattering the glass. The attack took place despite what was reported to be a huge security presence.
The BBC reports at least 30 dead. Wikipedia in a poorly written update to her page reports 80 dead. The Associated Press includes eyewitness reports of over a hundred wounded. The Telegraph reports at least 50 dead with the death toll expected to rise.
Pakistani security services had requested Bhutto to travel to Karachi by helicopter because of security concerns.
Bhutto had previously stated that she believed she would be killed if she returned to Pakistan. Numerous islamist groups had made death threats.
“I am not scared. I am thinking of my mission,” she had told reporters on the plane on her way into Pakistan. “This is a movement for democracy because we are under threat from extremists and militants.”
Background 
As an educated woman accustomed to power who seeks to modernize Pakistan, Bhutto is reviled by the Islamists. At the same time, she is one of the most popular figures in Pakistan, even after her popularity has taken a huge hit after she accepted a deal with newly re-elected and unpopular President Musharraf. Bhutto was removed from office due to corruption and embezzlement charges. A Swiss investigation into the charges (the money was allegedly transferred to a Swiss bank account) is due in the next few weeks.
Bhutto’s main base of support comes from loyalty to her father and her modernizing position. Hundreds of thousands gathered in Karachi and around the country today to welcome her back from exile and show support.
Wikipedia has a solid but brief bio.