Using movie clips in a conference presentation
Let's just preface this with 'I am not a Lawyer' (I am a Sociologist interested in copyright and piracy).

Fair-Use in the U.S. supports educators using small amounts of copyrighted material for educational purposes, and this includes to peers for conferences and workshops, so long as that material was legally obtained.

Copyright (in the majority of cases) gets invoked at the point of distribution so putting those clips into a .ppt and then sticking it online is not protected as you're now distributing copyrighted material without permission.

I would use the clips live, and then put placeholder images in the slides for the upload. If clips are available from reputable hosting sites like YouTube or Vimeo you can always include a link in the slide. Technically in the U.S. this can be framed as second order copyright infringement under 'inducement' (i.e. you are inducing, or encouraging someone to access copyrighted material illicitly by providing a route to it). 

However with all copyright it's a matter of enforcement, and more likely if the material is unlawfully available on a streaming site, the rights-holder will pursue the site, not the linker.