A report published by the Canadian Institute for Health Information has released a study on head injuries in Canada that found that the number of head injuries suffered by cyclists has decreased in the 10 year period from 1994 to 2004. The report states that the largest decrease was in children and youth between 5 and 19 years. The fact that other types of cycling related injuries have increased indicates that it is a significant decrease in this type of injury as noted in this quote from the report.

While cycling injury admissions (excluding head injury) have risen over the decade of study (from 3,512 to 3,790), admissions for head injuries have reduced significantly. Overall, head injuries due to cycling have significantly decreased between 1994-1995 and 2003-2004 from 1,507 to 815 (46%). The decrease in head injury admissions is seen most noticeably in children and youth (55%), specifically those between the ages of 5 and 19 years. However, there has been little or no decrease in cycling related head injury admissions in the adult population, and specifically no improvement seen for those aged 50 to 75 years.
(Head Injuries in Canada: A Decade of Change (1994-1995 to 2003-2004) - August 2006) - pg. 17

In provinces with province wide legislation, there is a 17% decreased risk of hospitalization due to head injuries when compared to provinces without legislation.
(Head Injuries in Canada: A Decade of Change (1994-1995 to 2003-2004) - August 2006) - pg.13

You can read the full report at the following link Head Injuries in Canada: A Decade of Change (1994-1995 to 2003-2004).