Psychological research on memorability of passwords?
It looks like there is some empirical evidence that passphrases are easier to remember than either random passwords (not surprising) or user-selected short passwords after standard advice to use mix of upper and lower and avoid words.

http://homepages.cs.ncl.ac.uk/jeff.yan/#exp

Thanks Jeff for the pointer in a comment (same Jeff?).


  J. Yan, A. Blackwell, R. Anderson and A. Grant. The memorability and
  security of passwords -- some empirical results. University of
  Cambridge, Computer Laboratory Technical Report No. 500, 2000. (see
  also Hungarian translation). PDF
  
  J. Yan, A. Blackwell, R. Anderson and A. Grant. Password Memorability
  and Security: Empirical Results. IEEE Security &amp; Privacy, Vol. 2 No.
  5, 2004. PDF


the above is reprinted as:


  J. Yan, A. Blackwell, R. Anderson and A. Grant. The Memorability and
  Security of Passwords. Refereed book chapter in Security and
  Usability: Designing Secure Systems that People Can Use (ed. by Lorrie
  Cranor and Simson Garfinkel), OReilly &amp; Associates, USA, 2005.