Each of the Employer/Author(s) retains the following rights ... The right to post author-prepared versions of the Work covered by the ACM copyright in a personal collection on their own home page, on a publicly accessible server of their employer and in a repository legally mandated by the agency funding the research on which the Work is based. Such posting is limited to noncommercial access and personal use by others, and must include the following notice both embedded within the full text file and in the accompanying citation display as well ...
In my area of CS at least, people nearly always hosted their own papers. Finding a free copy of a paper was never a problem -- it was just a matter of entering the paper's title in scholar.google.com, and clicking the first "edu" link that came up, instead of IEEE or ACM. This was faster to do than spending the 15 s switching on the VPN connection to the campus network.
I suppose students / professors hosting their own papers might have been unique to the area of CS I was studying...
[1] http://www.acm.org/publications/CopyReleaseProc-1.26.10.pdf