"Today’s bombing in Iran has ignited a lot of speculation as to who, or what organization is responsible. Within Iran, speculation will no doubt focus on The Opposition Movement of the Iranian People, also known as Jundallah." A World Without Spin
The site does an excellent analysis of who it may or may not have been that conducted the suicide attack, without jumping to unproven, unprovable conclusions in response to:
"A bomb in a mosque in south-east Iran has killed at least 15 people and wounded 50, the governor of Sistan-Baluchestan province said." BBC News Online
Other news sources are placing the death toll at between 20-30 and it could rise higher but some key information is that it was a Shi'a Mosque in a Sunni area of Iran on the Afghan-Paki border. Al-Qaeda is
The upcoming elections are cited as a motivation but the Ayatollah Khameni personally approved every candidate and has the final approval and 51% of the vote regardless of how the election turns out. For this to be a motivation, it would have to be believed by the people that there was really a choice to be made in the election.
Al-Qaeda is unlikely because they have ties to Hezbollah and the Iranian government. Even if they differ on who should run the caliphate they set as their mission or how and where to attack US Citizens, they share tactics and goals as well as recognize that Iran is a great diversion of international community to their actions. Frankly, they recognize Iran as much more akin to an ally than an enemy, particullarly when it comes to the "enemy of my enemy" strategy so prevalent in that part of the world.
On the other hand, Al-Qaeda and Hezbollah rabidly attacked the supporters of the other to foment violence in post-invasion Iraq. In the end, it was Iraqis that rose up against their "own" overlords in defense of their newly attained democracy and freedoms that Iran and Al-Qaeda fear.
While I find it ironic that terrorism has struck against the primary state-sponsor of terrorism, I cannot rule out that Iran itself is to blame for the attack which has the hallmarks of their tactics and allowed it to arrest "the terrorists" behind it. More than once, they attacked their own in Iraq through surrogates of Hezbollah, specifically to foment violence against others.
Tyrants and Terrorists think nothing of "sacrificing pawns" to further their own power. That is the reason that Hezbollah and particularly Al-Qaeda each kill more Muslims than any non-Muslim group.