I got hit with that one a short while back. A Village Atheist in an internet forum was spouting off the tired "All babies are born Atheists" meme, and then he tried to defend the meme with that comment.
Non-belief isn't a default, ignorance is. Belief or lack of it is a conscious decision.
For millennia, belief in an eternal universe was the "default position". In the early 20th Century, it was discovered that the Universe was finite. The "default position" was wrong, proving that it isn't inherently true.
In fact, the default position is often "ignorance".
The fact that God may be outside of our ability to prove His existence doesn't validate the "default position".
For Atheists, their "default position" is faith-based.
Ouch. I know how much Atheists hate to hear that.