Since the beginning of the 2009 season, Lance has been a .260/.384/.474 hitter, with 37HR, 167K:153BB. His OPS+ has been 128 - still pretty good, but not the Lance we were used to for nine full seasons.
Let's look at some numbers.
|Time Frame||PAs||Avg/SLG/OBP||K:BB rate||XBH%||HR/PA%|
So what the heck is going on? A lot of it can be chalked up to luck. From 2000-2008, Berkman's BABIP was at .300 or above - including a 2008 season in which his BABIP was .341. In 2009, his BABIP was a then-career low .296, and it's .274 as we stand right now.
While he's been remarkably consistent with his K:BB ratio, and his XBH rate, he's also striking out more often. In 2007, Berkman struck out 22.3% of the time. That dropped to 19.5% in 2008, rose to 21.3% in 2009, and is currently at 23.5%
For Berkman, the decline started rather abruptly at the 2008 All-Star Break. In the first half of the 2008 season, he hit .347/.443/.653, and followed that up with a .259/.384/.436 line in the second half. That's an OPS dropoff of 275 points, with a BABIP swing of 72 points (.370 before, .298 after).
This is interesting, too: While Berkman's HR/PA and XBH% rate have remained largely consistent, FanGraphs says that his ISO (SLG-AVG) is at an all-time low (save for the 1999 call up) - .181 this season. With his ISO topping out at .306 in 2006, he has posted .232, .255, .235, and .181 since.
So I'll open it up to the floor for conclusions, but it seems to me that Lance is going through a stage where the Pendulum of Luck has been swinging the other way, and he's just losing some power. In other words, he's simply declining.