This is an update to this post from the end of last season: The Phillies' Home: Fairest in the Land.
2008 - 2010
In summary, that article reviewed Park Factors (explained below) for all MLB stadiums for the 3-year period 2008-2010. It found that contrary to its reputation, and in spite of relatively short dimensions, Citizens Bank Park was one of the most neutral parks in baseball, both for overall scoring, and home runs in particular:
i.e. CBP boosted scoring by 1.5% (11 parks boosted scoring more), and it boosted HRs by 4.2% (11 parks were more homer-friendly).
The money quote:
... the bottom line is there are essentially no more runs scored, and no more home runs hit, in the Phillies' home games than there are in their away games.
Explanation of Park Factors from the Maple Street Press 2011 Phillies Annual:
Park Factors are calculated by comparing stats in a team's home games with stats in that team's away games, against the same opponents, in order to control for team-by-team differences. A number higher than 1.0 indicates a hitter's park, while lower than 1.0 is a pitcher's park -- a park factor of 1.1 indicates the stadium increases offense by 10%.
In the first half of 2011, the results have been very similar to the past 3 years. CBP is squarely in the middle, suppressing scoring slightly while being almost exactly neutral in terms of boosting or suppressing home runs:
(more after the jump)
2008 Through 2011 ASB
Adding the data from the first half of 2011 to the 2008-2010 results gives us these Park Factors for 2008-2011:
Note: NYM and NYY are for 2009-2011 only; MIN is for 2010-2011 only
and the same data, graphically (click to enlarge):