I set up sshcontrol file like so:
4788C56270A2DA4406CDD8B008C3DAA5F4268F03 36000 confirm
This works and confirmation is requested whenever I use the key (on OpenPGP card) to
authenticate to a server.
However, confirmation is _not_ requested on agent forwarded requests - e.g. if I login
to server_1 and then from server_1 to server_2, no confirmation is requested for the
second login but the key is still used.
I think this is a pretty serious bug as it allows forwarded keys to be stolen by anyone
with access to the machines I login to (with agent forwarding).
I use gpgtools on OSX, though, so there is a slight chance this is not a gnupg bug but
a gpgtools bug.