ActiveClient has an API. You can install it from the installation media. With that you could create a custom form that utilizes it. I'm not aware of any office API that will allow you to modify the default behavior for handling digital certificates.
I would stay away from it if possible, especially if your shop is not managing the client configurations. We've had way more issues troubleshooting client policies and configurations than it was worth. If your deployment is configured for PKI authentications already, and you restrict username/password logon, than the access stamps provided by SharePoint should be adequate. You can even take that information and feed it to InfoPath as a suedo signature.
Other CONS:
If you use multiple signature, it can take minutes to open a form as the signatures are validated. We have a form with 4 Signatures and it takes over 5 minutes to validate, so test heavily before relying on them. this is not an InfoPath specific item, as PDF and Word forms will do the same thing.
When a cert if revoked or expired it shows up in the form as invalid. So if you are looking for something to store for long term user, this may not be it, as new users get CAC/PIV's there old signatures will no longer be valid when the form is viewed.
If you want to validate an entire form, you can only do it via the thick client. Browser forms only do form sections.
