Perhaps this is too obvious to state, but progressive enhancement obviates most of this. Any linkable state (that someone might plausibly want to link to), should be addressable via a regular old URL. Add a layer of JS to intercept clicks and serve ajax content, with the state expressed in the hash.

The hash should look suspiciously like the fragment that non-ajax agents would see.

I say this not as a dogmatic should, but because I find separating concerns makes it easier to develop, and preserves web-ness. Ajax is an enhancement, not an architecture.