Should I worry that a graduate student somewhere gave a talk using my abstract?
I think there are basically two cases here:


The student is presenting or leading discussion on your work as part of a "journal club" sort of series, and the abstract is your abstract because they're talking about your paper.
The student is baldly and ridiculously plagiarizing.


I think a good way to approach this is to assume case #1, and make contact with the student.  You might email the student (paper trail), cc'ing both their advisor and yours, with something like:


  I noticed online that you gave a presentation on my paper [preprint link].  I'm interested to know how it was received, and if there was any interesting feedback.  Would you mind sharing your notes?


Now, if it's case #1, you've made positive contact with a possible fan and collaborator, and something really nice might come out of it.  If it's case #2, though, you've established a paper trail and made certain that the relevant supervisors on both sides are aware of it, so that if any incident happens in the future, your position is exceedingly clear and strong.