Is it ethical to submit a paper with the name of a co-author who cannot be contacted? The paper in question is based on research my collaborator X and I did together some months ago. Our respective contributions to the research were around 2/3 for X and 1/3 for me. In terms of the actual text of the paper, X wrote about 1/3 and I wrote about 2/3. 

During the writing of the actual paper, X decided to leave academia. I can no longer contact X: emails to the old address go undelivered and there is no forwarding address for physical mail. Attempts to find X through web searches and contact X through mutual acquaintances have been unsuccessful. X has thus not seen a complete draft of the paper, only the sections that were actually written by X.

In these circumstances, is it ethical for me to submit a paper with X's name on it, without a complete version having been checked by X and without X's approval?

If I do submit it, should it contain a note of the fact that X was unable to check the completed paper? I am considering the hypothetical possibility that I could have inadvertently introduced an error while writing the complete paper (and such an error might survive through peer-review and into publication); responsibility for such an error should be mine alone. 

There is no possibility of separating out my contribution into a separate paper. Either the work has to be published as a whole or not at all.

I am in a field where alphabetical listing of authors is standard, so there is no question of the order of authors.
This is actually the subject of a blog post over at Adventures in Ethics and Science. In general, it seems that several possible actions are defensible. However, whatever action you take, you should make sure to 


Document your efforts to reach your co-author
Let the journal editor know at submission time what's going on


RetractionWatch is full of stories papers that were retracted for having been submitted without a co-author's knowledge. These steps can help you avoid that fate.

Given you've done 1 and 2, your choices are then:


Include the co-author, despite the radio silence, or
Acknowledge the co-author


The problem with the first choice is that authorship signals endorsement of the paper's contents. If the missing co-author has not read and endorsed the final contents, having his name as an author is misleading. However, you can resolve this to some degree by including an explicit statement (e.g., in a footnote) that Author X could not be reached to review the final version of the paper.

The applicability of this choice also depends very much on the journal’s official policy and what responsibilities each of the persons listed as authors have met. I'm sure we can all agree that it would be out of bounds to forge the missing co-author's signature on a form that needs to be submitted with the manuscript! However, even if you have to sign a form stating that all the authors have reviewed the manuscript, you shouldn't do that either - tell the editor you can't, and explain why.

The problem with the second choice (acknowledgement instead of authorship) is that the missing co-author has presumably done enough to warrant authorship of the paper. However, you could argue that an essential part of authorship is to see the paper through to publication - and that if the missing co-author has not done that, he does not deserve authorship. Again, if you do this, you must clearly document the author's contribution in the acknowledgements and alert the editor.

Which action you choose will probably depend on authorship standards in your field and the outcome of your conversation with the editor. Whatever the result, make sure you clearly and honestly communicate the contributions (and lack thereof) of the missing co-author to both your readers and the editor.