Apple Support, in a pretty terse document:
The eligibility period for the MacBook Air (Original, Late 2008, and Mid 2009) Hinge Quality Program has ended.
The eligibility period covering this issue has ended.
I had forgotten about this situation, but I linked to a support article about it back in 2009.
The URL I linked to back then now redirects to the short announcement quoted above, but here is what it said:
Learn what you can do if you experience one or more of these issues with the hinges on your MacBook Air.