First, everyone needs to get this straight. Boldin is NOT playing out his last year of his contract. The only way he plays is with a new contract. So, if the Cards plan on keeping him and using him, he would need a new contract.
From Boldins point of view, he has $2.75M left on his contract. If he plays without a new contract, he risks injury and future worth for only $2.75M. If he holds out and gets a new contract he will end up with $7-$12M in upfront bonuses. If the Cards try to force him to play, he will hold out as long as reasonable (reasonable = until the fines get too large or until he needs X amount of weeks to have this year count as an accredited season). Once he is forced into playing, he can always "pull a hamstring" and maybe wait until the last 4 games of the year to "turn it on". If he does nothing all year, but has 4 good games at the end of the year, he will still command a very good contract in 2011.
With the WR depth on the Cards, I see no reason to offer Boldin a new contract, therefore, he must be traded.