Native tethering on Nexus 4 on AT&T: do they charge extra?

Hi there,

I am a huge fan of my new Nexus 4. I'm using it on AT&T's cell network with an old grandfathered unlimited data plan. I have what I believe to be a simple question: if I use the Nexus 4's native tethering feature, will I be charged extra on my bill by AT&T, or will they be none the wiser to my tethering activities? Also, would they try to move me to a limited data plan? I welcome both speculative and experienced posts.

Thanks in advance.