I'm doing some research about this topic and I can give you some hints, but I've not found a way to make it work yet.
Monkeysphere seems a very interesting project, but I've not been able to compile it under Mac OS X without clogging my little free diskspace with MacPorts.
The first way I suggest you to try is to generate a compatible authorized_keys entry from your key id (e.g. BFB2E5E3) with
gpgkey2ssh BFB2E5E3 |tee -a ~/.ssh/authorized_keys
Here I added it to my localhost since I ran an ssh server for testing purposes, but of course you should add this to the target host ~/.ssh/authorized_keys.
Next you need to tell SSH to use the private portion of this key during authentication, but simply exporting an ASCII armored version of the keypair doesn't work
gpg --armor --export-secret-key BFB2E5E3! |tee ~/.ssh/id_rsa
gpg --armor --export BFB2E5E3! |tee ~/.ssh/id_rsa.pub
chmod 400 ~/.ssh/id_rsa
gpg-agent has the option "--enable-ssh-support" that allows it to use it as a drop-in replacement for the well known ssh-agent.
I've read of some people trying to add via ssh-add their GPG key after launching gpg-agent this way:
gpg-agent --enable-ssh-support --daemon
gpg --armor --export-secret-key BFB2E5E3! |tee ~/.gnupg/exported-keys/BFB2E5E3_sec.asc
But I don't think this will ever work. The manpage says: "SSH Keys, which are to be used through the agent, need to be added to the gpg-agent initially through the ssh-add utility. When a key is added, ssh-add will ask for the password of the provided key file and send the unprotected key material to the agent; this causes the gpg-agent to ask for a passphrase, which is to be used for encrypting the newly received key and storing it in a gpg-agent specific directory."
So it seems that gpg-agent should be used as an additional measure to protect your SSH keys with a GPG encryption.
Converting a GPG key to OpenSSH
Jérôme Pouiller in his blog writes that Gpgsm utility can exports keys and certificate in PCSC12 and than used by OpenSSH using gpgsm:
gpgsm -o secret-gpg-key.p12 --export-secret-key-p12 0xXXXXXXXX
openssl pkcs12 -in secret-gpg-key.p12 -nocerts -out gpg-key.pem
chmod 600 gpg-key.pem
cp gpg-key.pem ~/.ssh/id_rsa
ssh-keygen -y -f gpg-key.pem > ~/.ssh/id_rsa.pub
But I haven't find a way to make gpgsm accept my gpg keypairs.
Other things you can try
SSH has -I option to specify the PKCS#11 shared library ssh should use to communicate with a PKCS#11 token providing the user's private RSA key.
ssh-keygen can use RFC4716/SSH2 public or private key, PEM PKCS8 public keys, and PEM public keys to generate an OpenSSH compatible private (or public) key using -i and -m options.
Still I miss a way to put it all toghether