Can't Digitally Sign a SharePoint Infopath Form with Internet Explorer 8

We ran into a bizarre problem when attempting to sign a SharePoint Infopath Form using an internally issued Digital Certificate. When the user clicks on Select Certificate button to sign the form nothing happens. Normally the user is prompted to select a certificate to sign the form. Also a snippet of the form should be displayed, but a red x appears instead of the form snippet.

This is an Internet Explorer 8 specific issue. The code that allows you to digitally sign the form is executed from a dll that is installed on the workstation. Fortunately the fix is very simple. In IE8 Click on Tools, Internet Options, Security Tab, Trusted Sites, Sites and add in the site about:internet to your Trusted Sites. For larger installations, you could automate this task by creating a Group Policy Object that adds in this site automatically for all domain users. Now you should be able to digitially sign the Infopath form.

It took us awhile to figure this out, hopefully this post will save you some grief!