On Mondays the team websites of MLB do their mailbag thing. I only checked a couple of the teams but all had questions about whether their team would be able to trade for Willis and/or Cabrera.
The Texas Rangers had this offering. The Somewhere in California Angels posed this question. Even the Marlins website got in on the action. While this isn't an official site mailbag, SI answered the call for the Mets.
Every site said that neither would be traded and I believe that to be correct. Willis and Cabrera will start the season with the Marlins but whether they finish it with the club is another matter.
Next year will be the 2007 season and that is when the lease expires on the stadium deal. If the Marlins organization doesn't have a stadium deal in place or has very little hope of one being reached in the near term, it is possible that one of the two players would be traded at the end of the season.
Neither player will become a free agent until after the 2009 season but they are probably going to command about half of the team's payroll together which could make them trade bait for a boat load of prospects.
Hopefully, this won't happen, even if a new stadium isn't in sight but it is in the realm of possibilities.