Upon being notified of the matter, the organization mounted an investigation, and has finally revealed its results:
The incident related to the communication of user IDs and passwords between two specific applications within our internal network resulting in the inclusion of such data in web logs.
An anomaly occurred with a process executed in coordination with a proxy provider of IEEE, with the result that copies of some of the logs were placed on our public FTP server. These communications affected approximately two percent of our users. The log files in question contained user IDs and accompanying passwords that matched our directory. The primary logs were, and are, stored in protected areas.
Upon discovering this exposure, IEEE immediately removed those files, ceased receiving those log files from the proxy provider, and corrected the interapplication communication that resulted in the logs containing user IDs and passwords.
The affected user accounts were locked down, and only affected users were notified that IEEE is requiring that each affected user change his or her password. Institutional account information was, and remains, unaffected.
IEEE made also sure to note that it does not store its corporate directory information in the clear, does not expose it to the public, and that the corporate directory hasn't been compromised.