Transient Parasthesia. Sounds pretty nifty, eh? Well, apparently that is what I have been suffering from since Heartland. According the book "Fixing Your Feet: Prevention and Treatments for Athletes" written by John Vonhof:
Transient parasthesia is a temporary nerve-compression that can be caused by a grudual buildup of fluids in your feet during extended on-your-feet activity. As the feet swell and blood flow decreases, nerves become compressed. During the compression, the nerves do not receive the oxygen-rich blood they need, resulting in numbness and/or tingling.
The numbness can last anywhere from several weeks to up to a year. It's been a month since Heartland, and I've still got a little numbness concentrated in both big toes. Ah, the fun of the ultrarun.