Is this an issue with conferences? My guess is that conferences would prefer to have interesting topics, and probably wouldn't care as much if the material was partially recycled. I have known people who presented early results of a study at one conference, and the final results at another. They were upfront about it, so it wasn't unethical, but I guess you could accuse them of resume padding. Personally, I would prefer a bunch of really interesting presentations at a conference with some recycled material, rather than a bunch of boring presentations that are all original (although if you attend a lot of conferences with the same people, you might prefer the latter I guess).