This might have been big news a couple of years ago when Andrew Miller was still supposed to make something of himself. Miller, 25, was 1-4 with an 8.54 ERA for the Marlins last year, and was pretty bad in the minors too. He had 28 strikeouts in 32.2 innings. He's still young, so it may be too early to call the former first round draft pick (6th overall) a bust, but he has pitched for parts of five seasons without anything close to success.
It cost them Dustin Richardson, who, despite bouts of wildness looked like a pretty decent option to be a left-handed reliever out of the pen next year. He did take a major step backwards last year though after showing considerable promise in 2009.
I do not do not know how to feel about this trade right now, but I guess we will see. The Red Sox did not really give up much and it is possible that at the least, Miller will be the left-handed relief option that Richardson would have been. Miller does have some high upside as a former top draft pick, so he could be salvageable.