In fact, has anyone ever seen a job post like this? I know I haven't; I'd remember. It's highly unusual in its straightforwardness. I'd guess that's pretty appealing to the kind of hacker they're looking for.
There are some very good arguments for keeping the information closed, but we thought we'd try it. So far we've had very positive reaction, but the real test is internal dynamics as the company grows.
A somewhat related anecdote... I recall with chagrin the first startup I joined. The CEO was a friend of mine. I kept asking him how much equity he was offering as a percentage. He kept responding evasively by snowing me with all sorts of dealy jargon that I didn't understand. In retrospect, of course, he was concealing the fact that the percentage was negligible. At the time, though, insecure in my knowledge of business, I walked away thinking that I was a pathetic noob for having the idea that percentage was a meaningful metric. (This guy was good at what he did, which was deceive smart but gullible people.) That experience is one reason I like your job post. I totally agree with the commenter who said that they hate the jockeying of the negotiation process and would find it a relief to just know up front.
Anyway, I find your post to be an interesting experiment and I think many of us would like to see a follow-up report on it someday.
I haven't heard a good argument in favor of hiding stock option percentages - I can't think of any other than concealing the fact that your share is negligible.