From the commit message:

    Added a gpgkey option to the account settings

    This allows the user to specify the gpg key used. In addition, if
    gpgkey is not set, and there is only one email address defined, then
    sup will not pass any id to gpg, so gpg will use its default key.
    Only if gpgkey is not set and there are multiple email addresses
    will sup use the old behaviour of defining the key to use by
    passing gpg the from email address.

This has been requested before:
http://rubyforge.org/pipermail/sup-devel/2009-November/000029.html

I've given it some basic testing (using the gpg-args hook to view the
arguments) and it appears to work as I intend it to in all 3 cases I
mention in the comment. I'm not a massively experienced ruby hacker,
so any feedback or improvements appreciated.

Hamish