First of all, I disagree that the negative time solution can be removed using math; the math will tell you that the solution is perfectly valid.
But you could, say, write a computer program that gave you the right answer to classical mechanics problems, right? In order to write this program, the knowledge you have that tells you that when you want a length of time, you want a positive number would have be translated into "computer language," i.e. math.
That is, when I say "you can remove nonsense solutions by using math" I mean "all you have to do is make the theory already contain your knowledge of what's a nonsense solution."
http://www.scottaaronson.com/blog/?p=1103
Eliezer's gung-ho attitude about the realism of the Many Worlds Interpretation always rubbed me the wrong way, especially in the podcast between both him and Scott (around 8:43 in http://bloggingheads.tv/videos/2220). I've seen a similar sentiment expressed before about the MWI sequences. And I say that still believing it to be the most seemingly correct of the available interpretations.
I feel Scott's post does an excellent job grounding it as a possibly correct, and in-principle falsifiable interpretation.