There is a big difference between "neutral" and "standard." There is no such thing as a "neutral" accent. As far as the US goes, there is, however, the American Standard accent. This accent can be heard from Katharine Hepburn, for example, but it not commonly spoken today. It is not associated with a certain region, and is considered a heightened way of speaking that you will pretty much only hear from theatre performances or students who have access to a dialect coach.

There is also a British Standard, though I'm not as familiar with it. I believe it is similar to RP, but I think there may be some slight differences. However, this accent is the "standard" British accent, just as American Standard is ours. (Sorry, Aussies, I have no clue where you come in to all of this.)

If you were to look at the accents across the US, it is probably true that Californians in general have the closest accent to American Standard, though I believe traditionally the "New England" accent used to be the closest (I've never heard anyone nowadays with that accent, so that may just be "historically speaking.") However, the general "Californian accent" is not neutral. It's just closer to Standard.

Nah, its cool, us Aussies all pretty much have a standard accent.. The differences between states is very subtle.. so. yeah. Haha

I don't know..there is like a 'continuum' from very Bogan Australian to educated Australian, I think.