Hi Crispin,
Thanks for reporting this, I confirm it as a bug. As you know most custom user profile properties are stored in the mp_UserProperties table but ones in the mp_Users table to have to be specifically wired up, and I could have sworn that I already did wire up the DateOfBirth field but looking into it apparently I did not do it. I will fix this today so it will be fixed in the next release.
Sorry for the delayed response, my Dad passed away recently and I have been offline quite a bit and this is the first chance I have had to look into this.
Thanks,
Joe