Mexico is difficult to tell what happened because they repealed their year old helmet law on the day they launched their bikeshare scheme. So too many things changed too quickly or at the same time to divine much I'm afraid. But if you want something up to date have a look at the recent Sydney study by Rissel and Wen on "The possible effect on frequency of cycling if mandatory bicycle helmet legislation was repealed in Sydney, Australia: a cross sectional survey" in Health Promotion Journal of Australia 2011; 22: 178-83. As you will probably need a subscription to view it I have copied the results of the study below.
Results: One in five (22.6%, 95% CI 18.8-26.4%) respondents said they would cycle more if they did not have to wear a helmet, particularly occasional cyclists (40.4% of those who had cycled in the past week and 33.1% of those who had cycled in the past month). Almost half (47.6%) of respondents said they would never ride without a helmet, 14.4% said ‘all the time’, 30.4% said ‘some of the time’ and the rest were not sure. One third (32.7%, 95% CI 28.5-37.0%) of respondents did not support mandatory helmet legislation.
Conclusions: While a hypothetical situation, if only half of the 22.6% of respondents who said they would cycle more if they did not have to wear a helmet did ride more, Sydney targets for increasing cycling would be achieved by repealing mandatory bicycle helmet legislation. A significant proportion of the population would continue to wear helmets even if they were not required to do so.