The forthcoming EndNote X3 will be compatible with OpenOffice.org Writer.
Here's an excerpt from the press release:
The Healthcare and Science business of Thomson Reuters today announced compatibility between EndNote®—the bibliographic management software used by millions of researchers, librarians and students—and OpenOffice.org Writer. The new EndNote X3 for Windows, which will be released in June, introduces the patent pending Cite While You Write™ technology to OpenOffice.org Writer, an open-source word processor popular among academics.
OpenOffice.org Writer support in EndNote X3 enables users to format in-text citations and the bibliography instantly as required by publishers. Users can search EndNote libraries within OpenOffice.org Writer and select references to cite. The OpenOffice.org Writer formats a paper automatically upon inserting an EndNote citation and can revise an entire paper using more than 3,300 different styles with no additional typing required.