When writing an abstract for a talk on a topic that I have collaborated on with others, should I be using the plural form, such as
"We have shown this and that ..."
or is it better to write it as:
"I will discuss this and that ..."
Use "we", because although only one person will be giving the talk, you are representing the work of multiple people.
The subject of course depends on the sentence, and in math either type of sentence is fine. If the subject is who is talking, use first person singular (you do not need to use the royal "we"). If the subject is who did the work, use the plural. Often I will vary the sentence structure so both "I" and "we" appear in the abstract.
Of course, you should certainly say who the work is joint with (if it does not automatically appear in the speaker info). E.g., "I will discuss joint work with Charles Xavier, where we..." Or just add a sentence to the end "This is joint work with Charles Xavier."
In general, I agree with jakebeal for the reason he gave. However, one of my colleagues was recently criticized for not using "I" in an abstract he submitted for a postdoc position. Apparently, some people like to see "I" as a way of differentiating between what was contributed by the person vs. what was done as a group (e.g., "I built the apparatus. We gathered data and analyzed it."). From your question, it's not clear in what environment your talk will be given, but in case it is indeed a job talk, you may consider using "I" at least once. Otherwise, I think "we" is appropriate.