The authorized keys files on both machines are the same. So, I am unaware of any other thing to check ..
On the one that works (host A->B) I get ..
debug1: Authentications that can continue: publickey debug1: Next authentication method: publickey debug1: Offering DSA public key: foo.macbook.air debug1: Server accepts key: pkalg ssh-dss blen 433 debug1: Authentication succeeded (publickey).
On the one that doesn't work (host B->A) I get :
debug1: Authentications that can continue: publickey debug1: Next authentication method: publickey debug1: Trying private key: /home/foo/.ssh/id_rsa debug1: Trying private key: /home/foo/.ssh/id_dsa
/etc/ssh/ssh_config files are the same on both hosts. and my
~/.ssh/authorized_keys is identical on both hosts, with permissions 600.
~/.ssh also has permissions 600.
Is the problem potentially that public key is not being 'Offered' by host B for some reason? I am at a loss as everything works one way, but not the other, and I can't see any obvious differences between the two servers.