British researchers report that on the weekends when the last two books of the series came out, young people made far fewer visits to an Oxford emergency room. The study, led by Dr. Stephen Gwilym of John Radcliffe Hospital, appears in the final 2005 issue of the journal BMJ, which tends toward the tongue-in-cheek in its year-ender. ...
The effect, it turns out, was significant. The researchers looked at how many children ages 7 to 15 went to the E.R. with musculoskeletal injuries on the 2003 weekend after "The Order of the Phoenix" was published, and on the 20o5 weekend of "The Half-Blood Prince." They compared these numbers with admissions in a three-year period.
On the Harry Potter weekends, they found, the admission rates went down by almost half - even though each was a pleasant summer weekend when business in the E.R. would ordinarily be good.
As reported in the New York Times. Here's the original article (also not very long).