They automate the process for obtaining DOIs for cited articles. See

"In a separate process, the publisher also submits the citations contained in each deposited article to the Reference Resolver, the front-end component of the MDDB that allows for the retrieval of DOIs. This way, the publisher can, as part of its electronic production process, add outbound links to any of an article’s citations that point to content already registered in the CrossRef system. The CrossRef website includes technical specifications for querying and a demo of the DOI look-up process. If you know the DOI for an article, that’s all you need to know in order to locate it persistently. If a publisher changes the location of an article, it need only update the URL for the article in one place with CrossRef."

Terry Bucknell
Electronic Resources Manager
University of Liverpool

-----Original Message-----
From: Code for Libraries [mailto:[log in to unmask]] On Behalf Of Jonathan Rochkind
Sent: 17 November 2009 23:50
To: [log in to unmask]
Subject: Re: [CODE4LIB] Assigning DOI for local content

Bucknell, Terry wrote:
> Note that as well as registering DOIs for the articles in LLT, LLT would be obliged to link to the articles cited by LLT articles (for cited articles that have DOIs too).

Huh, I didn't know that. I understand the motivation, but investigating 
whether every cited article has a DOI and then making sure to include 
it... is non-trivial labor.