I was kinda waiting for FiveThirtyEight to weigh in on this: using Benford's Law to check for fraud in the Iranian election results (here as well).
Benford's law is sometimes useful in these cases, because human beings intuitively tend to distribute the first digits about evenly when they're making up "random" strings of numbers, when in fact many real-world distributions will be skewed toward the smaller digits.
Both 538 pieces are skeptical that Benford's Law is applicable in this case. (thx, nick)
Update: Voting fraud expert Walter Mebane has produced a paper on the Iranian election that uses Benford's Law to check the results. He's updated the paper several times since it was first published and now writes that "the results give moderately strong support for a diagnosis that the 2009 election was affected by significant fraud". (thx,scott)
Update: Done just after the election, this analysis shows that the returns released by Iran's Interior Ministry during the course of the day of the election shows an unnaturally high steadiness of voting percentages. (thx, cliff)
Update: Regarding the previous link, Nate Silver doesn't think much of that analysis. (thx, cliff)