How do I cite / write about something someone has just said during a presentation?
I'd use the form for symposia, adjusted slightly to reflect that it's a keynote speech with only one speaker. 

For the APA style, see the second block in this blog entry:


  Symposia
  
  A symposium is a bit like a live-action edited book: Several authors come together under the leadership of the chair to pool their knowledge about a topic.
  
  Krinsky-McHale, S. J., Zigman, W. B., &amp; Silverman, W. (2012, August).
       &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;Are neuropsychiatric symptoms markers of prodromal Alzheimer’s 
       disease in adults with 
  &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;Down syndrome?
   In W. B. Zigman (Chair), 
       Predictors of mild cognitive impairment, dementia, 
  &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;and mortality in 
       adults with Down syndrome. Symposium conducted at the meeting 
       of the
  &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;American Psychological Association, Orlando, FL.