I found a thread on the Calibre forum that suggests it's possible to create a BibTeX file from within Calibre (which Zotero can import):

http://www.mobileread.com/forums/showthread.php?t=108952

(also, some documentation on OPF: http://idpf.org/epub/20/spec/OPF_2.0.1_draft.htm )

Does this still work for people? When I try to import the *.bib catalogue I exported from Calibre, Zotero hangs for me (that is to say - it starts the dialogue box, I see a bit of green going left and right, but then it just seems to freeze); and when I use the java-script posted here, Zotero complains the file format isn't right. Any ideas? Thanks much.

Sure:
<XML><RECORDS>

<RECORD><REFERENCE_TYPE>1</REFERENCE_TYPE><REFNUM>1</REFNUM><AUTHORS><AUTHOR>Schember, John</AUTHOR></AUTHORS><YEAR>2010</YEAR><DATE>Jun</DATE><TITLE>Calibre Quick Start Guide</TITLE><PUBLISHER>calibre</PUBLISHER><LABEL></LABEL></RECORD>

<RECORD><REFERENCE_TYPE>1</REFERENCE_TYPE><REFNUM>2</REFNUM><AUTHORS><AUTHOR>Koren, Yoram</AUTHOR></AUTHORS><YEAR>2010</YEAR><DATE>Jul</DATE><TITLE>The Global Manufacturing Revolution (Wiley Series in Systems Engineering and Management)</TITLE><PUBLISHER>Wiley</PUBLISHER><ISBN></ISBN><LABEL></LABEL></RECORD>

<RECORD><REFERENCE_TYPE>1</REFERENCE_TYPE><REFNUM>3</REFNUM><AUTHORS><AUTHOR>Hussain, Imtiaz</AUTHOR><AUTHOR>Pattnayak, Satya R.</AUTHOR><AUTHOR>Hira, Anil</AUTHOR></AUTHORS><YEAR>2008</YEAR><DATE>Jun</DATE><TITLE>North American Homeland Security: Back to Bilateralism?</TITLE><PUBLISHER>Praeger Security International</PUBLISHER><ISBN></ISBN><LABEL></LABEL></RECORD>

<RECORD><REFERENCE_TYPE>1</REFERENCE_TYPE><REFNUM>4</REFNUM><AUTHORS><AUTHOR>Fransman, Martin</AUTHOR></AUTHORS><YEAR>2010</YEAR><DATE>Jul</DATE><TITLE>The New ICT Ecosystem: Implications for Policy and Regulation</TITLE><PUBLISHER>Cambridge University Press</PUBLISHER><ISBN></ISBN><LABEL></LABEL></RECORD>

<RECORD><REFERENCE_TYPE>1</REFERENCE_TYPE><REFNUM>5</REFNUM><AUTHORS><AUTHOR>Andersen, Torben Juul</AUTHOR><AUTHOR>Schrøder, Peter Winther</AUTHOR></AUTHORS><YEAR>2010</YEAR><DATE>Jul</DATE><TITLE>Strategic Risk Management Practice: How to Deal Effectively with Major Corporate Exposures</TITLE><PUBLISHER>Cambridge University Press</PUBLISHER><ISBN></ISBN><LABEL></LABEL></RECORD>

<RECORD><REFERENCE_TYPE>1</REFERENCE_TYPE><REFNUM>6</REFNUM><AUTHORS><AUTHOR>(Editor), Jonathan Reuvid</AUTHOR></AUTHORS><YEAR>2010</YEAR><DATE>Jun</DATE><TITLE>Managing Business Risk: A Practical Guide to Protecting Your Business</TITLE><PUBLISHER>Kogan Page Ltd</PUBLISHER><ISBN></ISBN><LABEL></LABEL></RECORD>

<RECORD><REFERENCE_TYPE>1</REFERENCE_TYPE><REFNUM>7</REFNUM><AUTHORS><AUTHOR>Gray, Cheryl</AUTHOR><AUTHOR>Hellman, Joel</AUTHOR><AUTHOR>Ryterman, Randi</AUTHOR></AUTHORS><YEAR>2004</YEAR><DATE>Jun</DATE><TITLE>Anticorruption in transition 2: corruption in enterprise-state interactions in Europe and Central Asia, 1999-2002</TITLE><PUBLISHER>Washington, D.C. : World Bank, c2004.</PUBLISHER><ISBN></ISBN><LABEL></LABEL></RECORD>

<RECORD><REFERENCE_TYPE>1</REFERENCE_TYPE><REFNUM>8</REFNUM><AUTHORS><AUTHOR>WetFeet, </AUTHOR></AUTHORS><YEAR>2008</YEAR><DATE>Jun</DATE><TITLE>Deloitte Consulting</TITLE><PUBLISHER>WETFEET, INC.</PUBLISHER><ISBN></ISBN><LABEL></LABEL></RECORD>

<RECORD><REFERENCE_TYPE>1</REFERENCE_TYPE><REFNUM>9</REFNUM><AUTHORS><AUTHOR>Pigman, Geoffrey Allen</AUTHOR></AUTHORS><YEAR>2007</YEAR><DATE>Jun</DATE><TITLE>World Economic Forum: A Multi-Stakeholder Approach to Global Governance</TITLE><PUBLISHER>Routledge</PUBLISHER><ISBN></ISBN><LABEL></LABEL></RECORD>

<RECORD><REFERENCE_TYPE>1</REFERENCE_TYPE><REFNUM>10</REFNUM><AUTHORS><AUTHOR>Schwab, Klaus</AUTHOR><AUTHOR>Roubini, Nouriel</AUTHOR><AUTHOR>Bilodeau, James</AUTHOR></AUTHORS><YEAR>2006</YEAR><DATE>Oct</DATE><TITLE>The Financial Development Report 2008</TITLE><PUBLISHER>World Economic Forum</PUBLISHER><ISBN></ISBN><LABEL></LABEL></RECORD>

<RECORD><REFERENCE_TYPE>1</REFERENCE_TYPE><REFNUM>11</REFNUM><AUTHORS><AUTHOR>Donnelly, Thomas</AUTHOR></AUTHORS><YEAR>2005</YEAR><DATE>May</DATE><TITLE>The military we need: the defense requirements of the Bush doctrine</TITLE><PUBLISHER>Washington, D.C. : AEI Press, c2005.</PUBLISHER><ISBN></ISBN><LABEL></LABEL></RECORD>

The javascript was in the mobileread link posted by Rintze in this thread - here is the URL: http://www.it.usyd.edu.au/~tapted/bib2endnote.html - and this is a BibTeX to Endnote (proprietary XML) converter.

Well as I said, Zotero hung when I tried to import the *.bib file into Zotero. So I figured I had the same problem that made the other people try the java program. I'll try again

Still no solution here? I don't know about you guys, but I LOVE calibre for managing my ebook collection, and I LOVE zotero for managing my e-article collection (and also the books that I actually end up citing). Will the twain then never meet?

OK, I have identified the origin of these silly publication dates (Jan. 1st, year 101): that's the metadata I have in my calibre database for some e-books, probably a default value assigned by calibre when a specific date isn't available in the imported document and designed to stand out when sorted in ascending or descending order.

So as far as I can tell: exporting metadata from calibre to Zotero through the Bibtex format works just fine.

Here is what Kovid Goyal says:
"click the arrow next to convert books and create a catalog in bibtex format. endnotes [and zotero] ought to be able to import a bibtex catalog".
----
yes, it works on me.
one more question: is it automatically updating when calibre's library grows?