The Sox boast possibly the best top 3 of a rotation in the game of Beckett, Lester, and DiceK. I've already talked about the first two the other day, so a word about DiceK. He's tough to watch, and one really wonders when and if his luck is going to run out, in that he puts a lot of guys on base but pitches out of it. Last year he loaded the bases 15 times... and opponents batted 0.000 against him. If you assume that the league bats 0.300 with the bases loaded (it is actually higher than that), then you'd get this outcome (0.7)^15 = 0.5% of the time. That's a good a definition of luck as you're going to find. My main beef with DiceK is that all his nibbling and high pitch counts leave him unable to consistently go deep into games, and the Red Sox bullpen (all teams' bullpens, really) pretty much stinks. But the guy won 18 games last year and had a sub 3.00 ERA, so all in all, he's solid.
The backend of the Sox rotation is still in flux. Wakefield may or may not be back next year -- the tea leaves point towards retirement for the longest-serving current member of the Red Sox (since '95), but my gut tells me he'll be back. Wake is kinda a mixed blessing, as he's no longer the guaranteed-2oo-innings guy he once was. If he's your number 5 starter, fine, but even as a 4 that doesn't make for a very deep staff. Other arms to consider to round out the rotation are Clay Buchholz and Justin Masterson. The former stormed onto the scene with a no-hitter in his second MLB game in 2007, but was less-than-impressive in '08, ending the year in the minors. Masterson began the year as a starter for the Sox but eventually moved to the bullpen, much more a commentary on the sorry state of the Sox bullpen rather than as a demotion for his work as a starter. By the time of the playoffs, Masterson was essentially Fracona's fireman, the get-me-out-of-trouble in the middle innings guy out of the pen.
If I had to guess, I think the Sox make a play for a free agent pitcher to slide into the #4 slot -- Ben Sheets would be my first choice, because I think he's got a lot of upside, but Derek Lowe appears to be an option as well. The 5 spot is then Buchholz's to claim, as all things being equal, I'd prefer to see Masterson stay in the pen.