At first, I thought maybe Howard was striking out less or walking more, which would explain the drop in power by an adjustment in how he plays the game. However, after looking at his stats, he's striking out near his career rate (61 in 224 AB) while walking far less (18 in 242 PA).
So, I then started to look at ancillary concerns:
Weight -- For a 6'4 player, Howard is overweight. Think about this for a moment. When Howard decided to lose weight, he was 275 lbs. By contrast, when people called Miguel Cabrera fat, he was 240 lbs. At the end of the 2009 season, Howard was placed on a strict regiment of organic foods and lower fat meals to help with that problem. But, even with significant weight-loss (over 20 lbs), he is still heavier than "fat" Miggy.
Age -- Howard is 30 (and turns 31 shortly after the season ends). While he has averaged nearly 50 HR per season over the last four years, all four years were in the middle of his prime. 30 is generally considered to be the start of the "decline" phase for not just athletes, but the population at large. Although Howard probably benefits from his workouts, his unhealthy diet for the first 30 years of his life (the meal regiment changed around his 30th birthday) will be difficult to overcome with just six months of lifestyle changes. More than likely, Howard will play at a viable (no longer peak) level until 33, at which point, those 30 years will catch up to him.
Team age -- The average age of a Phillies batter is 32.1. This is the highest that it has been since 1983. With the team (on average) past its prime, Howard does not have the necessary protection in the lineup to be a consistent power hitter.
Barry Bonds -- During Spring Training, Howard sought out the tutelage of the greatest hitter of his generation (yes, even without the PED boost late in his career, Bonds would have had this designation). What did Bonds tell him? Stand closer to the plate. This is horrible advice for Howard. In the past, pitchers would be able to pitch Howard to the outside corner of the plate because he wouldn't be able to reach those pitches. As a result of moving closer to the plate, Howard is able to "work the count" significantly better than in years past (after 0-1: 2010 -- .293/.346/.455, 2007-2009 -- .211/.275/.455; after 0-2: 2010 -- .205/.295/.359, 2007-2009 -- .127/.165/.288).
However, Howard doesn't have Bonds' tool set or his eyes. As a result, Howard is LOSING ground on counts that favour the batter (after 1-0: 2010 -- .283/.359/.446, 2007-2009 -- .294/.441/.632; after 2-0: 2010 -- .217/.455/,217, 2007-2009 -- .316/.591/.665; after 3-0: 2010 -- .250/.750/.250, 2007-2009 -- .310/.853/.483).
When you combine the components, it's no wonder that Ryan Howard is struggling at the plate (both in terms of power and OBP). Barry Bonds took a power-hitter who has really bad eyes and forced him into a box where those with good eyes excel.
Given that the majority of these issues are not within Howard's control, he should fix the one thing that he can: move away from the plate. As much as he might think otherwise, he is NOT Barry Bonds and does not have the tools (vision and pitch-selection) necessary to excel in a Bondsian style of play.