It appears that when the Houston Astros officially come under new ownership, likely at next week's MLB owners' meeting, they will join a new league as well. According to an Associated Press report quoted on the Houston Chronicle's Ultimate Astros blog:
Astros owner Drayton McLane, buyer Jim Crane and Major League Baseball have reached an agreement on shifting the franchise to the American League, the Associated Press reported on Friday.
The move, if it happens, would reportedly take effect in 2013; spokespeople for all parties involved either declined comment or pled ignorance on the subject.
The move would balance out the two leagues and shrink the largest division in Major League Baseball, which would all likely pave the way for a move to two wildcard teams from each league. The last team to change leagues was the Milwaukee Brewers, who made the opposite move (from American to National League) in 1998.