It's quite surprising that the IS hasn't struck in Iran earlier and heavier. They kill Shiites by large scores in Iraq. They also kill Sunnis by large scores, in many places, merely for not being as crazy as they are.
The West needs to understand that we are actually on Iran's side as far as the really big terrorism problems go, and that the Arabian dictatorships are at least as much our enemies as Iran is.
It's easy, and appears too plausible, to suspect Saudi Arabia to somehow be behind this attack, but at this point that would amount to a cheap conspiracy theory, IMO. KSA is somehow behind the IS, but probably not on an operational level, more as a sponsor and spiritual guidance thing.