Not that great, signed messages in Outlook have a visible attachment, which
is unfortunate as it makes signed email unusable to me (clients complain
that they can't distinguish emails from me that have explicit attachments,
from the rest, when all of my emails have the paperclip icon, by virtue of
being signed.)

The disturbing thing is, OE does it right, signed emails do not appear to
have a spurious attachment. Perhaps even more disturbing, I have to assume
that spurious attachment in Outlook is by design, as it has persisted across
at least 4 versions of Outlook.  Could they be unaware that this usability
flaw is putting users off of signed email?

Anyway, whether the attachment is accessible I couldn't say, but if it is,
try using CAPICOM to extract details from the attached cert.  (CAPICOM is a
crypto API COM object, freely available on MSDN.)

-Mark