I hope this isn't too late.
the militarycac.com site is really great for getting configured. I'm an Army Reservist, and for the longest time, I had to bootcamp into Windows (ugh) to use my CAC reader. (I couldn't get Parallels to fly - something about the virtualization and the reader has a hiccup).
Just recently (Sep'11), I've tried using my reader (MBP, Snow Leopard) to access AKO, etc. It worked! I had a PDF that needed a digital signature: Preview=no joy. Adobe Reader=Worked!
I have not tried to sign an XFDL file yet, but will do shortly.
Big issue is the upcoming restriction of only allowing access to email with a CAC. I think I'll be ok on my MBP, but my iPad is a different story. A USB reader can't seem to connect via the camera adapter thingy and I'm not going to buy the bluetooth sled. If anyone has gotten the USB reader to work, please spread the love.