That is exactly what Psychohistorian expected.
Also, we don't know that there was "no bias at all" for taking the survey, just that the net change in bias between the first group and the second group was relatively small. I expected there to be a big bias in who made it through the first filter (reading the original article and finding out about the poll in its last paragraph) and multiple additional biases which would partially cancel out, and predicted that the net effect of these additional biases would be for the second group's AQ to be a bit lower than the first group's (but still substantially higher than the true population AQ). For instance, only some of the nonresponders from the first round actually saw your request for them to take the second-round survey, and they might tend to be high-AQ because they needed to look at your second Aspergers post to see the request.
I made this prediction about the AQ scores with low confidence, since it's hard to guess the relative sizes of all of these potential biases, or even to identify every relevant bias. For instance, the data are showing that there were slightly more people who have been diagnosed with Aspergers in the second round than the first, and this could reflect a genuine difference (rather than random variation) caused by another bias: people who have been clinically tested for Aspergers might have been more likely to read the post but less likely to go on to take the survey, since they had less to learn from it.
My main point in this discussion is that, for future surveys, it's better to try to avoid selection effects in the first place than to try to account for them after the fact, since they can introduce a lot of uncertainty which is hard to get rid of.
Yeah, you actually predicted that the second responders would be slightly less AS-ish than the first responders, but actually they are slightly more.
To be honest, this result surprised me too, I expected that the AQ scores would go down, so I'm updating towards the "no large net bias" hypothesis.
Followup to: Aspergers Poll results
Since my little survey about the degree to which the Less Wrong community has a preponderance of people with systematizing personality types, I've been collecting responses only from those people who considered taking the survey after looking at the original post, but didn't, in order to combat nonresponse bias.
82 people responded to the initial survey, and another 186 responded after the request for non-responders to respond. In the initial survey, 26% of responders scored 32+ (which is considered to be a "high" score, and out of a group of Cambridge mathematics students, 7 out of 11 who scored over 32 were said to fit the full diagnostic criteria for aspergers syndrome after being interviewed).
In the combined survey of 82 initial responders and 186 "second"-responders, this increased to 28%. In the original survey, 5% of respondents said they had already been diagnosed with aspergers syndrome, and in the combined survey this increased to 7.5%.
Overall, this indicates that response bias is probably not significantly skewing our picture of the LW audience, though, as always, it is possible that there is a more sophisticated bias at work and that these 268 people are not representative of LW.