Not quite sure this is true about the ACM. From their policy about rights permanently retained by the author [1]:

* Post the Accepted Version of the Work on (1) the Author's home page, (2) the Owner's institutional repository, or (3) any repository legally mandated by an agency funding the research on which the Work is based

* Post an "Author-Izer" link enabling free downloads of the Version of Record in the ACM Digital Library on (1) the Author's home page or (2) the Owner's institutional repository;

[1] http://www.acm.org/publications/policies/copyright_policy#Re...