rlange - 1:43 pm on Aug 9, 2011 (gmt 0)
It seems that the X-Purpose header is sent for "Google Instant" requests, not for "Google Instant Pages" requests. There is even an unresolved bug on this submitted in the Chromium project - [webmasterworld.com...]
OK, now I'm confused. What is it about Google Instant that needs to make a request to a website? I was under the impression that it was just an "instant search results" feature.
But I think you're right; that information I linked to above is for Google Instant, not Google Instant Pages (curse Google and their naming nonsense).
From the issue you linked to...
At this point we're still gathering developer feedback about prerendering (including this need) and haven't made a decision.
That was nearly 2 months ago now. They must have decided on something if they've made the feature available and on-by-default in a stable release.