As of Acrobat 6, it is possible to add a visible signature block to a document
when a digital certificate is used to sign a document. When showing OOo's
digital signature facilities to govt authorities here in Brazil, I was asked it
it would be possible to add such a feature to OOo.

The functionality would be the following:

Digitally signing a document would allow the user to add a "signature object" to
the text that would reflect the validity of the signature. This object should be
printable in order to indicate that a printed copy was made from a valid signed
document.