Actually, no. In Australia upon introduction there was no meaningful decrease, and since then, cycling rates have dramatically _increased_. For example a 3x increase in the last decade in the largest city:
The article also links to some peer-reviewed research that shows that while less-dangerous injuries have been going up proportionally with bike usage, the level of head injuries is way down as a result of the helmet legislation.
Slashdot Mirror
The IRA were terrorists too, and a large proportion of their funding came from the US. Turns out that somehow doesn't count.
http://www.smh.com.au/opinion/society-and-culture/bike-helmet-critics-not-using-their-heads-20121003-26yvc.html
The article also links to some peer-reviewed research that shows that while less-dangerous injuries have been going up proportionally with bike usage, the level of head injuries is way down as a result of the helmet legislation.