There's a draft <GL3/gl3.h> header in the Registry now, as described in appendix H.2 of the OpenGL 3.1 specification. This is not ready for general use yet, but we'd appreciate feedback from interested parties, particularly in terms of making it work with existing GL 3.1 implementations (getting feedback from Mesa/X.org folks is probably most critical in this regard).
Eventually it will be possible to just drop gl3.h in on top of a shipping GL3.1 implementation, but to get there we need to do some work on the calling conventions and other platform-specific stuff in the header before it's likely to work well on most platforms.
In terms of the interfaces provided, it should be pretty good, but I wouldn't be surprised if a couple of tokens and functions either should be there and are not (core 3.1 functionality), or shouldn't be there and are (removed functionality).
None of the preprocessor logic should be taken too seriously at this point other than GL_VERSION_3_1. Much of it is leftover from rewiring the glext.h generator scripts and there's no
reason to continue #defining GL_VERSION_3_0 (or previous), for example. There are also some extraneous typedefs, such as GLintptrARB, that are only meaningful with glext.h and will be removed soon.
It would be really helpful if feedback came by way of Khronos Bugzilla rather than in the forums, since TBH I am so swamped with internal Khronos stuff that I rarely have time to keep up with the forums. If you do provide feedback that way, please use product "OpenGL" component "Registry".