Who might the Giants get and how?
Two CFs that come to mind as trade bait that I've seen in the press before are Mike Cameron and Ken Griffey Jr. Both teams had looked at trading them at one point or another though currently there is no talk floating around in the press. And both are with teams with more outfielders to play than they have spots in the field and could benefit from changing their excess of outfielders into other good players - the Mets have Victor Diaz who did well in Cameron's absence while he was on the DL. And both players could probably use a change in scenery, Cameron wants to play CF, Griffey might thrive again in a new situation, he might have worn out his welcome in his home town. In addition, Sabean often does these type of blockbuster trades when the team needs a boost, basically to shock the patient alive again.
Obviously, Cameron would seem the most desirable of the two in the viewpoint of the Giants management, solely because of Griffey's salary and remaining contract length. However, the Mets are probably not willing to eat Cameron's salary since they have been using him in RF OK, whereas the Reds could be desperate to get rid of Griffey because they are obviously rebuilding and his contract is a big albatross on the team's next 5-7 years (whatever his length of contract left). And they have great young outfielders spliting time, Austin Kearns, Adam Dunn, and Wily Mo Pena, plus Ryan Freel plays out there on occassion as well. They might be willing to eat part of the contract, mainly most of this year's salary and parts of future salary, a la the A's trade for Jason Kendall, where the Pirates got rid of their albatross contract by paying a lot of his future salary.
What can the Giants give in exchange? Well, one of their aging outfielders to help even out the salary the Giants take on, and I guess if they get a CF, either Tucker OR Grissom could go, so maybe it could be Grissom going after all, though I think the Giants will still try to keep him. And definitely prospects, though not Cain, but perhaps Hennessey might go if the Giants think one of their other starter prospects are ready to move up to the majors or, if not ready, ready enough to learn at the major league level (i.e. Cain or Valdez). I would assume that Ellison, Niekro, and Linden can be made available as well, depending on what the other team wants and how they structure the salary and contract of the CF. That's because we have a number of up and comers who could play those positions (Lewis, Ishikawa, and Ortmeier/Schierholtz/EME come to mind) as starters in 2-3 years when those positions need replacing.
Thinking more about it, Hennessey seems to be a good place to start. Hennessey has done OK in the time he has spent in the majors plus he's won a few games already. More importantly, he has pitched good this season in his three starts. In addition, Fassero had done well as a spot starter and Brower also did well in a starter's role. So perhaps the Giants trade Hennessey and then to replace him, one of these two keep the starter's seat warm for the first Giants pitching prospect to prove himself in the minors, whether Cain, Valdez, Misch, Foppert, or whoever (oddly enough, Cain does not show up on the Grizzlies stats page in MILB.COM, do they know something...).