Here's Idra's distilled argument:
1) Slow Overlords can only scout certain builds successfully as a result of failures on the other guy's part (misplaced buildings, failure to position anti-Overlord units correctly)
2) Some of the aforementioned builds are impossible to counter without taking huge risks unless they can be scouted before they leave the P/T base
Idra's expressed desired outcome was that Zerg scouting be buffed or that the race have a "build that is safe against everything." People are getting really hung up on Idra's use of that particular phrase and it's easy to see why: if there were really one build that Zerg could do against everything, that would be quite imbalanced. However, I give Idra the benefit of the doubt and interpret the phrase a little differently. (If I'm right, then his hyperbole doesn't do his argument any favors, but it's a highly emotional topic and these things happen.)
I think what Idra really wants is simply for it to be physically possible for a Zerg to live through a given build having made every reasonable effort to scout said build as early as possible and having not taken too many risks with his own build. Given the current design of the game, that means making it possible for Zerg to scout those builds more quickly or making it possible for Zerg to defend those builds if the P/T build is scouted only after it moves out.
A lot of people are getting hung up on that step, as well, but one has to realize that just because it would be possible to defend a build doesn't mean it will be easy. Idra wants things to be easier for himself, of course, but kicking the difficulty down a notch from "literally impossible" (which is how Idra sees it) doesn't mean that it's suddenly going to be "so easy a bronze newbie can do it." In fact, it will most likely leave Idra as one of the few people who can pull it off even somewhat consistently.
Day9's distilled argument is, of course, pretty much a reaction to Idra's; he says that it's not yet empirically impossible that some Zerg will come up with a build that can flex to defend whatever moves out of the P/T base, and therefore Blizzard shouldn't get too hasty with the balance changes.
As far as I can tell, Day9's desired outcome (he doesn't state a desired outcome as clearly as Idra during the episode) is that the community continues to wait and eventually Zergs come up with builds that can react to a given P/T build at the earliest possible scouting point. Unfortunately, this is a really tough sell as it is both the solution that requires more time and the less emotionally-charged solution. There are a couple of really undesirable things that can happen as a result of taking the Day9 approach:
1) Eventually, no one plays Zerg. Day9 admits to the possibility during the episode
2) Blizzard (ignoring pro Zergs' anecdotal evidence entirely) gathers all the necessary empirical evidence to indicate what balance changes should be made, but fails to take the correct action. They aren't perfect, after all!
But all in all, it's another solid choice alongside the Idra option of "make balance changes now."
I hope this helps someone out there understand the whole picture better. I felt like I had reached at least some clarity on the topic after a few hours of thinking about it here and there, and I was still seeing forum posts on various sites that I felt grievously misapprehended the whole debate. Feel free to respond with your thoughts or questions, but please leave your emotions about Day9 and Idra as people out of it.