Sunday's stinker aside, Beckett's peripheral numbers are still pretty good:
K/9
http://www.fangraphs.com/graphs/510_P_season_full_1_20080818.png
BB/9
http://www.fangraphs.com/graphs/510_P_season_full_2_20080818.png
K/BB
http://www.fangraphs.com/graphs/510_P_season_full_3_20080818.png
WHIP
http://www.fangraphs.com/graphs/510_P_season_full_6_20080818.png
SO what's happened between last year and this year?
BABIP - He's been unluckier on balls in play
http://www.fangraphs.com/graphs/510_P_season_full_7_20080818.png
LOB% - Has undergone a correction from his brilliant last season
http://www.fangraphs.com/graphs/510_P_season_full_8_20080818.png
And probably the biggest reason - HR/9 has also undergone a correction
http://www.fangraphs.com/graphs/510_P_season_full_4_20080818.png
His FIP is 3.38 - this tells me he's been a bit unlucky this season. I don't find anything too alarming, he just needs to make some minor adjustments. Last season, he could get away with not throwing his off speed stuff for strikes because everyone was sitting fastball.
This season batters have adjusted - and Beckett hasn't quite adjusted back.
(For reference, in terms of FIP-ERA - Beckett is the 4th unluckiest pitcher in the majors behind Carlos Silva, Nate Robertson, and Luke Hochevar - but he's the only one with an ERA under 5)