Should I add references to conference presentations?
If the slides you're using are going to have "independent life,"—in other words, if you're going to make them available separately from the conference paper (on your website, for instance), then the citations should be included as part of the presentation. I would follow posdef's example and place the citations on the same slide as where it's needed; this will save the reader from having to flip back and forth between different parts of the presentation or between the presentation and the paper. 

Not including the citations is a bad idea, because it means you are potentially failing to give people the credit they deserve for ideas that were originally theirs. Even though it's "just" a conference presentation doesn't mean that the rules of crediting people for their work should be ignored. (Citing the work of others is also the right thing to do from the perspective of "playing nice with others." Taking credit for other people's work can make them leerier of working with you.)