(The point is: it's not easy to audit. Not that I actually care one way or the other.)
Easy == it wouldn't take a lot of work for him to allow someone to verify it.
Unless it becomes routine to actually employ these auditors, that doesn't really answer jtokoph's worry.
Postscript - Indeed, that's what he meant: on http://daltoncaldwell.com/we-did-it he says "In the very near future I will ask an impartial 3rd party take a look at our data (while preserving all privacy of our backers) and publicly verify that the join.app.net was operated in an honest manner."