Do you think it is against the academic code of conduct to present a paper at a conference or a departmental seminar that has already been submitted to a journal for publication?
There are two aspects to your question:
“Duplicating” a publication by having both a conference paper and a journal paper about it.
This has been discussed here before, and a short summary is that this usually not a problem: apart from Computer Science, where conference papers have a rather different status that in other fields, “publishing a paper does not normally prevent you from presenting your work at a conference”.
Presenting as-yet-unpublished results in a conference/seminar.
There's nothing wrong about that at all, and I think more people will actually enjoy you discussing recent results rather than old stuff. Customs, here again, depend on your field: I've seen more people discuss unpublished results in physics, and fewer in chemistry… but even in fields where it is less common, it is not forbidden.
One thing to note: if the paper is not yet published, it is probably polite to inform your co-authors (or ask for their permission, depending on your relationship) that you are going to present it at a conference. At least, that's what I do (and expect my co-authors to do).
In addition to F'x's answer:
it may be good to tell the audience that the corresponding paper was submitted/accepted and give the corresponding reference, so they can look up the paper once it is out.
In addition to F'x's answer:
I not only don't see this as a problem, but I think it is a great thing to do! By presenting a submitted (or not yet submitted) paper at a conference, you get instant feedback by a self-selected sample of people who were interested enough in your research to attend you talk - basically, the mother of all peer review. Of course, it won't be as in-depth as the reviews you get from the journal submission, but it can still be extremely helpful. And you can (and should) use the feedback from the conference to improve your submitted paper in the review process.
I do not know of any journals that do double-blind review. Having said that, here's a related scenario where there might be a problem.
You have submitted a paper to a conference that requires anonymized submissions, and you give a presentation at a public seminar on the material while the paper is under review.
Here, while there isn't an issue of misconduct, there's a sense of actively breaching the double blind guidelines. Again, there are caveats here: some venues are more relaxed about this than others.
While I think it is rarely an issue in the practical sense, I would still heed caution. Unless you are publishing your paper for open access, you are most of the time signing a publishing agreement with the journal and in so doing transferring copyright to them. While most journals have a liberal agreement when it comes to preprints, some do not. Hence, it is a good idea to check with your journal before you decide to publish something. Wikipedia has a helpful list of publishers and their preprint policies here.