An AFCI is a great addition to K&T wiring. While the conductors in K&T are separated by large distances, and even studs, they do come together at junction boxes which are often metallic. An AFCI adds a layer of peace of mind to the situation.
Note: when I replace a fixture in a K&T circuit I address this weak point with some new loom, slipped over the wires as they come into the box. The old loom, which could easily be 50 to 100 years old, is sometimes but not always brittle.
Be sure to measure your K&T wire to determine gauge. It can take slightly more current than the modern equivalent at the same wire size.
You'll probably need a double AFCI, as K&T was often wired with a shared neutral.
Fora more complete writeup of K&T retrofit see http://diy.stackexchange.com/a/20279/5960