If I don't belong to a registered theme camp...

[Don Gately]

Can I still set up my structure somewhere within A-K if I arrive on 8/27 at around 8 a.m., or are the inner arcs only available to registered camps?

[sputnik]

Some areas are reserved for theme camps, but the majority of the space is open camping. Find a spot with enough room and setup.

[Dork]

The areas in blue are reserved for theme camps:
http://www.burningman.com/preparation/m ... 07_map.pdf

Many of the other inner streets will already be taken by the time you get there, so getting a spot on A could be difficult. You may need to go back a few streets before you find something open. Wander around (on foot or bike is best IMHO) and talk to people standing near open spots.