While I agree that Dice-K is having a craptastic first half and shouldn't be taking the ball every fifth day, I think moving him to the bullpen to serve as an example for any pitcher who even thinks of screwing up shows an astonishing lack of forethought. Anyone who's a fan of this move does realize that we're stuck with Matsuzaka for another three years, right? If we throw him into the 'pen and he never gets a chance to recover the magic touch that made him a strong addition to the rotation in the last two years, those three years are a complete waste.
So, rather than bullpenning him, how about putting him on the DL? Pitchers - even pitchers with as many innings on their arms as Dice-K has - don't completely fall apart at age 28 unless they're hurt...and conveniently enough, Matsuzaka has already been on the DL this year. If he never fully recovered, it might very well be at the root of his pitching problems.