Excerpts from Andrej Podzimek's message of 2011-06-22 18:42:28 -0400:

Could I try your hack, pretty please? If there's any chance it could either resolve this problem

    http://www.mail-archive.com/linux-btrfs@vger.kernel.org/msg10683.html ,

or at least restore the data from the filesystem, then I'd like to give it a go. Waiting for the new btrfsck is currently not an option for me :-)

It looks like your box is failing to read the extent allocation tree.
We don't allow the mount to proceed without that tree, but you don't
actually need it for a readonly mount (to copy things off).

Josef, is your hack just a mount option to make -o readonly skip the
extent allocation tree?

I can put this into my -o recovery patch and we can give it a try.

-chris