From Hathihelper's developer:
"According to their March 2012 updates, HathiTrust data access is going to get more restrictive:
Over the next several months HathiTrust will be implemeting security enhancments to the Data API. The enhancements will require developers using the API to acquire an OAuth 1.0 access key that identifies them, and a secret key that must be used to “sign” URLs to retrieve HathiTrust resources via the Data API. HathiTrust will also provide a Web client that employ’s a user’s login credentials as a proxy for these keys to facilitate non-programmatic uses. In March, staff at the University of Michigan integrated 2-legged OAuth into the Data API and began to develop the Data API client. Once OAuth is released, there will be an approximately 6-month transition period, ending October 1, 2012, during which signed access to the Data API will be possible but not required. After October 1, all requests to the Data API will need to be properly signed with an access key retrieved from HathiTrust. Complete documentation of the security enhancements and methods of obtaining keys and accessing the Web client is forthcoming. OAuth is planned for release in April 2012.
You may want to complete any archival activities sooner rather than later. I will update my tools if there is an option for the hoi polloi to obtain access keys. I suspect that there won't be and my next iteration will have to be a rewrite that scrapes PDF from the web viewer instead of using a proper API." http://www.sciencemadness.org/talk/viewthread.php?tid=12438&goto=search&pid=245574