The short answer is that it does appear that boys and girls are entering puberty earlier, at least in the more developed Western world. The research on girls is more conclusive on this issue. However, scientists believe that the research showing boys are entering puberty earlier as well has been significant enough to warrant further investigation of the issue [Source: Tanner, Sofia].
A North Carolina study published in "Pediatrics" in 1997, showed girls at younger ages beginning to show signs of puberty, such as breast development or growth of pubic hair [Source: Miller]. More recently, a study conducted at the Cincinnati Children's Hospital Medical Center showed a growth in the percentage of girls beginning to show signs of puberty as young as 7 years old [Source: Yang].
The reasons for the earlier ages of girls entering puberty are varied. Researchers have found that childhood obesity can be an important factor in bringing on early puberty. Some of the early studies lead some to believe that increased exposure to certain chemicals, like phthalates or BPA, could be a cause [Source: Miller, Parker-Pope]. However, a recent study conducted at the University of California, Berkeley School of Public Health (published in the "Journal of Adolescent Health" in 2010) found that the absence of the biological father in upper-income homes is linked to early puberty in girls as well [Source: Yang] . One theory for this is that homes with no biological father may have a wider range of unrelated males in proximity to the girls, which triggers pheromones. Since exposure to artificial light has been connected to early puberty in animal studies, another theory is that girls in upper-income homes are exposed to more electronics.