John Maine being part of this deal is interesting, because Maine was one of the rumored pieces last year, when Billy Beane was shopping Tim Hudson and Mark Mulder to the Orioles.
If a Zito trade happens, I'd guess the scenario now looks something like this:
From Oakland to Boston: Jay Payton
From Boston to Oakland: David Wells (to replace Zito)
From Oakland to Mets: Barry Zito
From Mets to Oakland: Lastings Milledge, Victor Diaz (to replace Payton), John Maine
Whether something like that happens or not, we'll just have to wait and see. Beane certainly didn't slam the door shut on that possibility in his recent interview with Blez. Until Jeff Weaver and David Wells get new homes, we won't know for sure.
Update: Or maybe not. Looks like the Red Sox are about to acquire Coco Crisp, so they won't need Payton. Which might mean the Red Sox would want Justin Duchscherer for Wells, as previously rumored. That would mean that the A's wouldn't need Diaz, but that the A's would want Aaron Heilman instead, to replace Duchscherer. Mets GM Omar Minaya is probably reluctant to part with both Heilman and Milledge, so this deal is looking less likely now.