He has said elsewhere, in response to questions about his choice of fonts:

  This is a very funny question, why I use Comic Sans. So. All my talks use
  Comic Sans, and I frequently see little remarks, “Simon Peyton Jones, great
  talk about Haskell, but why did he use Comic Sans?” But nobody’s ever been 
  able to tell me what’s wrong with it. I think it’s a nice, legible font, I 
  like it. So until someone explains to me — I understand that it’s meant to be 
  naff, but I don't care about naff stuff, it’s meant to be able to read it. So 
  if you’ve got some rational reasons why I should not, then I’ll listen to them.
  But just being unfashionable, I don’t care.