The Twins are openly shopping Johan Santana, the best pitcher in baseball. Doesn’t it stand to reason that the Giants should be able to get a similar return for Matt Cain or Tim Lincecum? 
Here’s why. Santana is expensive. He’ll earn $13 million in 2008 then enter free agency. If his acquirer doesn’t want to extend his contract — which is likely to be more than $20–million-per-year for at least five or six years — they’ll end up trading a package of players for a one-year rental. Extend him with a new contract, and pay top dollar for a pitcher well into his 30s.
If you were a GM, wouldn’t you rather have Matt Cain? He isn’t as good as Santana right now, but he could be in 2009, 2010, and so on… at a much lower price and with less mileage on his arm. If Santana fetches the Twins a package of premium prospects, Brian Sabean’s hand will be greatly strengthened.
***
P.M. UPDATE: The Twins are close to trading a pitcher, but it’s not Santana. Instead they’re on the verge of trading young starter Matt Garza to the D-Rays for Delmon Young. Four other players could be involved.
What this means for the Giants: The Rays were one of the teams with whom the Giants seemed to match up well trade-wise. I spent my fair share of time wondering which outfielder the Giants could pry loose for Noah Lowry. Matt Cain-for-Carl Crawford has also been a common Internet meme. With the acquisition of Garza and departure of Young, Tampa Bay probably has no more need for what the Giants have.
Either Cain or Lincecum are years away from being able to garner a Santana-like package. JS is 29 - this is who he is and will be - one of the best pitchers in all of baseball. The risk factor associated with the acquisition of Santana is pretty much only injury-based.
Cain and Lincecum present equivalent if not greater injury risks and also present the risk of not panning out.
This issue of "mileage" is overrated and also cuts both ways. Santana has proven that he can pitch full seasons and post-seasons and be effective. Indeed, the performance of Fausto Carmona and CC Sabathia come playoff time illustrates the difference.
Would Cain or Lincecum garner a nice package via trade? Sure. But there is a reason why Cain and Lincecum are cheaper commodities right now, they are for the most part unproven.