I've been doing that too actually, although I am somewhat tempted to upvote some of the recent survey-takers just to make the playing field more equal for people whose other time commitments made them unable to take the survey very early.

I thought about suggesting to Yvain to edit his post by including a suggestion for people who have finished the survey to check back again later to upvote new survey-takers, but I get the impression he may prefer having this incentive against people procrastinating on taking the survey. It does at least mean that on average, the more heavily involved LWers are going to be awarded more karma since they're more likely to notice the survey as soon as it's posted.

This however has to be weighed against the disincentive for latecomers to take the survey if they didn't see or were otherwise unable to take the survey early. (Yvain has also on occasion made little changes to the survey after it's been posted, but I don't think that's enough to be a good incentive to take it later.)