Is there a module to assist with allowing users to log into a Drupal site using a common access card (PKI authentication)?
One of the cool things about PKI is that the authentication is handled outside of drupal so drupal doesnt need to actually care about PKI. That means that your choice of modules to use goes up quite a bit.
once someone is PKI authenticated, the webserver just puts a bunch of headers into each request that that an application (like drupal) can read and use.
If your requirement ends there, and any pki authenticated user can be a drupal user, and you have reasonable user attributes in the pki certificate, then just about any header-based authentication module will work.
While not technically built with PKI in mind, I'll bet that the siteminder module http://drupal.org/project/siteminder will work nicely for what you need to do.
Thanks Chris. We actually ended up writing our own module which seems to be doing the trick.
There's actually a module for this over at https://www.drupal.org/sandbox/rickwelch/1663258. The issue for promoting it to a full project is https://www.drupal.org/node/1775964.