Juan Cole makes a good case for it. At the very least, the numbers point to irregularities. Follow the comments for a range of opinions, as well as for what appears to be commentary from inside the country. Even if the election wasn't stolen, there seem to be a lot of Iranians who think it was--and that's what counts.
This could be the beginning of the fragmentation of the elite predicted by Bruce Bueno do Mesquita's models at the start of the year. From his TED talk: