Comments in unit tests should say "degrees," not "miles":
MongoDB docs are confusing, only the spherical queries $nearSphere and $centerSphere use radians. The $near and $center queries, which are the ones that are used in these mongoengine tests, use the same units as the coordinates. For distances on the earth, this means they use degrees. (This is also why the 2d indexes can be used for other 2d data, not just points on the earth.)
Here's a mongo shell session demonstrating that $near and $center use degrees, and that $nearSphere and $centerSphere are the ones that use radians: https://gist.github.com/964126
So, these mongoengine tests are using $near and $center, which use degrees, not miles, kilometers, or radians.
Thanks glyphobet for the clarification and taking time to explain it!
Ross Lawley