Hi,
I know this answer doesn't address your question but I thought I would just throw this out there - we have a lot of customers who find it exceedingly difficult to work with Digital Signatures. The whole certificate issue can be a mess and long term maintanence makes this a lot of work.
Due to these issues we came up with a unqiue solution to digital signatures - it basically uses an image stored in a SharePoint library that when a user wants to sign a form they input a pin (personal pin associated with that image) and the image from SharePoint is shown in the form.
For many of our customers who are not programmers this tool has been a life saver, you can take a look here: http://www.qdabra.com/en/products/ElectronicSignature.aspx
Ted