The current disc quota system is loosely based upon a very early scheme implemented at the University of New South Wales, and Sydney University in the mid 70's. That system implemented a single combined limit for both files and blocks on all filesystems.
A later system was implemented at the University of Melbourne by the author, but was not kept highly accurately, eg: chown's (etc) did not affect quotas, nor did i/o to a file other than one owned by the instigator.
The current system has been running (with only minor modifications) since January 82 at Melbourne. It is actually just a small part of a much broader resource control scheme, which is capable of controlling almost anything that is usually uncontrolled in unix. The rest of this is, as yet, still in a state where it is far too subject to change to be considered for distribution.
For the 4.2BSD release, much work has been done to clean up and sanely incorporate the quota code by Sam Leffler and Kirk McKusick at The University of California at Berkeley.