I'm guessing it's official support.  @joncotton has done some digging over in another part of the code on the commit rather than the PR -- those comments are relevant here though.

I'm ok with it so long as the tests are passing.  I have some pretty explicit tests verifying the `select_subclass()` behavior across Django 1.3, 1.4., and 1.5, so I'm pretty confident in it.