That Washington Post survey you mention is flawed for a number of reasons, the main being that while it showed Ahmadinejad ahead 2:1, it was only with 33.8% saying they'd vote for Ahmadinejad, 13.6% for Mousavi, and a full 42.5% of the respondents saying that they were either undecided or would not answer the question.
Obviously a survey in which the amount of undecideds is far greater than the amount saying they will vote for a candidate is hardly evidence of strong polling for that candidate.
Let's suppose, however, that 100% of the undecided went for Mousavi. Well, he would have just barely won, about 55-56%. It is statistically improbable that 100% went for Mousavi. So while we are not talking about something that was mathematically impossible, we are talking about something that was statistically improbable.
Washington Post estimated Ahmadinejad victory by margin of 2:1 about 3 weeks before the election based on polling data.
There are also rumors sifting out of the intelligence community that the margin was expected to be about 2:1.
Obviously the rumored intelligence report I cannot verify.
The Washington Post report is elaborated on here:
http://news.bbc.co.uk/2/hi/middle_east/8101841.stm (Just search the article text for 'Washington Post' if you are not interested in the rest.)
I'm starting to get the feeling that this was not a rigged election.