Giving a talk on a paper whose authorship is not finalized I will shortly be giving a seminar presentation on a research project that I worked on with collaborators X, Y and Z.  We have not yet submitted our paper for publication, and one question we have yet to resolve is the authorship - it is not yet decided whether Z will be a coauthor on the paper.  

(The reason is perhaps the reverse of what you might guess: Z doesn't think his contributions were sufficient to deserve authorship, but others of us think that they were.  We are still debating this with Z, and it will probably not be resolved before my talk.)

In my talk, I would of course like to give credit to my collaborators.  The usual way would be to begin the talk by stating that "this is joint work with [list names of coauthors]".  But I wonder whether to include Z in this list.  

If I mention that this is joint work with X, Y and Z, then the members of my audience would normally expect to see Z as a coauthor on the paper when it is published. If we end up not including him as an author, they may think that something strange is going on - like maybe we improperly denied him authorship, or he withdrew from the project over doubts about its quality.

But if I don't mention Z, I will feel like I am denying him credit for his contributions.  And I think it would be inappropriate to explain to my audience that Z's authorship status is undecided - the debate is a private matter among us collaborators.

Is there a good way out of this dilemma (or maybe trilemma)?
Z's name, I am sure, will be on the paper as an acknowledgement at least, even if not as an author. With that in mind, you could probably quite comfortably put Z's name up as a contributor without worrying about misleading the audience.

In some fields, this is made easier by a tradition of having an acknowledgements slide rather than putting up multiple names as authors on the first slide. In this model, only the speaker goes on the title slide, and everybody else worth mentioning goes in the acknowledgements slide (typically the last slide, which stays up during questions), with no particular distinction being made between levels of contribution. If this is accepted in your field it can give you a neat manner of sidestepping the question.