You should be open with what you want. It should not be shocking that you'd like to publish. Just be realistic as well. As a poster already mentioned, some doctors only have a single project or two on the side that they're working on, while others will have more project ideas than they know what to do with.
However, be realistic with your own level of expertise. If you have a lot of research and publication experience and can work semi-independently on a project and publication, you'll be much more useful to the good doctor than if he or she has to walk you through everything from the original project idea to how to submit your manuscript. It's a lot easier to give people discrete tasks (data entry, chart review) than to show them how to pull an entire project together, and he or she may not have the time or patience for the latter. Publications are easier to get in larger labs for that reason, there's usually plenty of grad students and post-docs around so you don't have to bug the PI to death with minor issues.