I presented my psychology thesis at an undergraduate conference, an international conference in France, and am now interested in submitting it to be presented at the 2013 APA conference in Hawaii. Since it's been presented to different audiences at each presentation is this unethical? Also, the first two presentations were simply poster sessions. If I elect to present the paper as a 10 minute talk rather than a basic poster session, would that be unethical?
Ask the conference organizers.
The ethics of multiple presentation, parallel submission, and multiple publication vary from field to field, and even from venue to venue within any field. Only the conference organizers (aka program committee or steering committee) can answer definitively whether they would encourage, allow, accept, discourage, or forbid speakers to present results that have been presented before.
If the conference has an explicit call for submissions, read it carefully; it may include specific language addressing this issue.
One thing you should absolutely not do is attempt to hide the fact that you've presented this result before. Do not seek forgiveness instead of permission. Even if the submission policy clearly allows you to submit a previously presented result, you should make its presentation history clear when you submit. If the organizers accept your submission even with this data in hand, you're clear!
It might depend on the field.
If there are no conference proceedings, there is no publication whatsoever. Therefore, it should be no problem to present the same work at multiple conferences. In fact, this is commonly done to reach a bigger audience.
If there are conference proceedings, the situation may be a bit more complicated. It depends on how proceedings papers are considered in your field. In my field, they're not really counted as anything at all, therefore it is still fine to present the same work at different conferences. But if they are counted and considered as relevant in your field, the situation may be different. In this case, I would recommend contacting the host of the session to ask for clarification.