Hi James and welcome to the InfoPath Dev forums.
While InfoPath does support digital certificates they can be quite a bit of work to maintain in your InfoPath solution. We have had many customers come to us with Digitial Certificate issues and what we have found is that what many customers need is the ability to have somebody sign a form electronically, using an actual digital certificate to do it is not required, it's the process in the form of having a way to securely sign the form.
We have actually developed a very cool way to handle this, it's a tool called "Qdabra Electronic Signature" and it's an InfoPath template part so you can re-use it over and over again in your InfoPath solutions.
You can read details about the product here: http://www.qdabra.com/en/products/ElectronicSignature.aspx
You might find our tool will meet your needs and could actually save you a ton of time in your InfoPath development
Ted