Eric, loaded question!
Actually, there was a thread on that one not so long ago (somebody might want to look it up and give the thread, but I'm lazy). Basically, it does vary from dojo to dojo, and from organization to organization. The dojo from whence I originate wears hakama only at the dan levels, and also wear white belts all the way through, even dan grades.
Other dojos in the same organization have different guidelines as to when to wear hakama.