I've exchanged emails with Ian Casselberry who runs the Tigers' SB site Bless You Boys.  I asked him for a scouting report on Marcus Thames as a firstbaseman.  Here's what he had to say:

I'm not sure I can give you a detailed report on Marcus Thames, but I - and I think almost every Tigers fan - has been impressed with the job he's done filling in at first base.  For an outfielder, he's shown a knack for snaring ground balls down the line and on in-between hops.  He covers the pop files in foul territory well (maybe his outfield experience helps there).  And overall, I think he's shown good instinct and athleticism at the position.  

Thames has had some struggles with footwork (there was a game in Minnesota where his foot was off the bag on a close play at first), and doesn't look very comfortable handling pick-off throws from the pitcher.  And I can't recall him starting a 3-6-3 double play or making any other play where he had to throw the ball.  

Perhaps the best endorsement is that Jim Leyland said he'd start giving Thames more playing time at first until Sean Casey begins to hit.  Considering that Casey's stayed in the lineup largely because of his defense, I think it shows that Leyland is comfortable with Thames playing there now and wouldn't hesitate to start him there in an important game against a left-handed pitcher.  

Now Thames is not a great hitter (his .242/.315/.474 is very nearly the definition of "league average"), but I was looking for someone who could be a useful utility guy for us.  
For comparision, Josh Phelps is hitting .269/.337/.470.  

Being able to backup first and the outfield would make Thames as useful as Phelps and Thompson while only using one roster spot.  Efficiency matter to me.

So what do the Tigers need that we have to offer?  Sadly, it looks like less than when I first dreamed this up a week ago.  At the time I was thinking Thames and Mike Rabello (their backup catcher) for Farnsworth and cash.  Since then, Joe Mesa has been cut to make room for Fernando Rodney.  
On our side, the injury to Minky and the probable move of Damon to first makes upgrading there less urgent.

I still think the Tigers could use the bullpen help (and we have Britton to replace Farnsworth), but Cashman might not deal with a team we're chasing for the wild card.

Anybody got a better idea?