If Billy were to trade Zito.....

who would be the likely suitors?

As you have all heard, Zito may be on the trading block with the trading deadline about a week away. The A's have arguably the hottest trading commodity with Barry Zito.
Billy, as we all know by now, will not trade Zito unless he gets something substantial with a young starting pitcher and some young offensive help.  It also has to be a deal that would improve the A's now, and in the future. It must also be a deal that would be better than the two first round picks that the A's would receive in the event that Zito leaves via free agency.  This limits what teams can or would trade with Billy. With the help of Baseball Prospectus and John Sickels website, I have put together an analysis of potential Zito suitors.
(Just a warning, my writing style is not as colorful as most other posters on this great website.)
Billy will not trade Zito to a team in his own division. He may trade to other divisions in the AL only if the other team would not become notably better than the A's in the event the two teams meet in the playoffs.  The Blue Jays may be the only potential suitor for Zito in the AL. But, the Blue Jays do not have blue chip AAA pitching prospects. Their pitching prospects include; Josh Banks, Chi-Hung Cheng, and Shawn Marcum. None of these prospects stand out any greater than Jason Windsor and Shane Komine. Trade unlikely.  
The National League, though, is a different story. the Giants, Padres, Diamondbacks, Dodgers, Reds, Astros, Cardinals, Braves, and the Mets are all potential suitors. Once again, any team that trades has to have a blue chip young pitching prospect that Billy could insert into the starting rotation, and other talent to give up in addition.  The Giants, Padres, Reds, and the Braves do not have such a pitching prospect.  That removes them from the list.  The Diamondbacks have great position players in their minor league system with Carlos Quentin, Stephen Drew, Justin Upton, and Conor Jackson, but no great upper level pitching prospects. The Reds have a great pitching prospect in Home Bailey, and a great OF prospect in Jay Bruce. Unfortunately, Bailey is tearing up AA and is not ready for the majors.
The Dodgers have a top pitching prospect with Chad Billingsley and a great AA pitching prospect in Scott Elbert.  The Cardinals have a top pitching prospect in Anthony Reyes, and a great OF prospect with Colby Rasmus.  Both are great players, with Reyes right now pitching in the majors. The Astros are back in the standings, but they may feel they have to make a bold move since some of their players are aging. The Astros have a decent prospect with RHP pitcher Jason Hirsh, but not much in other prospects. Hirsh is a big guy, but doesn't have great velocity.
And finally, the Mets have some prospects no one has heard of.....................that was a joke. The Met's top pitching prospect is Pelfrey, and a great AAA OF prospect with Lastings Milledge.

In summary, out of all of those teams, the Dodgers, Cardinals, and the Mets all have a young top pitching prospect that could step into a starting role with the A's.  Ned Colletti of the Dodgers is a good pitcher away from blowing away the Giants, and he may want to make a name for himself in his first year as Dodgers G.M.  The Cardinals have shown a willingness to trade top prospects in order to win now.(See Mulder trade). The Mets, though, are the most intriguing.  They have the right kind of prospects; the A's need a young, good, major league ready pitcher to fill Zito's shoes with Pelfrey, and some offensive help for now and the future with Milledge.  Also, the Mets have an aging pitching staff that is on borrowed time. Glavine, Hernandez, and Pedro aren't going to be able to perform at top levels much longer in this league.  G.M. Minaya may feel this is his best chance to win a world series with a lot of his offensive players having tremendous years.  He may take a serious risk in order to win it all, plus, he has the Braves breathing down his neck. Pitching coach may also be lobbying for Zito to come to the Mets.

If Billy were to decide to trade Zito, and that is a big IF, it would appear to me that Mets, Cardinals, and the Dodgers would have to best chance to get him. In my opinion.