After doing some digging myself here and other places, I also think this hole is for a badger den. If you start at the Esperero Trail entrance closest to the visitor center, you don't have to walk far to see this on the right. There is a mound of dirt in front of it (that doesn't show up in this photo) and the size seems right for a badger. Photo by Ned from 11/17/10. Badger tip by Jensine, Fred, and Ned.