UPDATE: There's been an armed attack on Ahmadinejad's campaign office in Zahedan, as well.
The bombing of a mosque in Zahedan, in the southeastern Iranian province of Sistan and Baluchistan, has reportedly killed 15 people and wounded another 50. Worshippers were gathered at the Amir al-Mohini Mosque for the observance of a Shi‘ite holiday marking the death of the Prophet's daughter Fatima. This is being blamed on Jundallah ("Army of God"), a shadowy Sunni movement active in southeastern Iran, where there is a fairly substantial Sunni minority in the majority Shi‘ite country. It is perhaps best known for a series of attacks in February 2007, but has been a persistent if low-level nuisance for some time. (For some background on Jundallah here's a Wikipedia entry; Pakistan has on occasion handed over arrested guerrillas to Iran, and Iran has charged that the US and Saudi Arabia support Jundallah, though the latter seems a bit more likely than the former.
So far as I am aware anyway, Jundallah's operations have been limited to Sistan and Baluchistan (one province despite two names), and particularly Zahedan, areas close enough to allow infiltration through the largely empty border areas with Pakistan and Afghanistan. It would be a mistake to see this as either a major indicator of Sunni dissidence in Iran generally or as a genuine threat to the regime. It is a regionally-based problem, but the bombing of a mosque is likely to lead to an intensified crackdown.