I had to skill up recently on the use of digital signatures in InfoPath, and this webcast I found is pretty amazing. I had no idea how rich the support in InfoPath for digital signatures was, and this webcast goes into *deep* detail. I found myself dozing off after about 45 minutes, but that was because I had what I needed by then. Still, it's a great resource.
The presenter is Mihaela Cristina Cris and she is a big brain at Microsoft on InfoPath.