Sonoran Alliance is encouraging everyone to get familiar with Article 5, Section 6, of the Arizona Constitution so we can all follow along with the order of succession if Napolitano decides to leave. If she does decide to leave, Secretary of State Jan Brewer would become governor and she would then be able to name her replacement as Secretary of State. Though not limited to pick someone of her own party, one would assume that Brewer would appoint a Republican.
Former Congressman and radio host JD Hayworth offered up an interesting and much more convoluted scenario which would yield a very different result. You can listen for yourself by clicking on this link and then going to about the 10 minute mark.
He illustrates a scenario in which Senator John McCain would be offered a position in Obama's "postpartisan" cabinet. If McCain took it, Napolitano would then appoint a Republican to fill his seat. Napolitano could then pick Secretary of State Jan Brewer. Then Napolitano would then be free to leave to join the Obama administation elevating Attorney General Terry Goddard to Governor.
To expand on this, Goddard could then appoint whomever he wanted to AG and Secretary of State. One would assume he would elevate Democrats to both of those positions.
While highly unlikely, it is an interesting scenario. However, this would be predicated on the fact that McCain would be willing to go work for Obama (which seems doubtful) and that the Obama administration would want two high profile Arizonans on the Cabinet.