You can access their paper here.  My knowledge of statistics is pretty grim, but nevertheless, I don’t have a good feel for this study:

Morimoto and study co-author Susan Chang, assistant professor of advertising and public relations at the University of Miami, randomly assigned 119 college students to a survey that asked about either spam or direct mail. On a scale of one to seven, where one is most intrusive and seven is least intrusive, students gave spam an average intrusiveness score of 1.93 compared to 4.24 for direct mail. For irritation, the average score was 2.46 for spam compared to 3.87 for direct mail. 

119 people?  Hmmm.  My view of direct mail as the greater ill ties in directly as to why the study may not tell the whole tale.  There’s one thing I care about with respect to mail – aside from packages – and those are bills.  Direct mail makes slightly more difficult and time consuming the task of sorting the important (financial) mail from the rest of the stuff.  College students don’t tend to deal with bills, or at least don’t view them with the same sense of importance as the older set.  This could explain why college students don’t view direct mail as the greater ill.  (I know of more than one college student who let the mail simply pile up or disregarded it.)  This, and the fact that college students spend more of their energy and time socializing (online).  Spam undoubtedly makes this activity more time consuming.  Particularly when you are communicating with people you don’t necessarily know (e.g., via myspace).

In any event, I find direct mail ten times more annoying than spam.  I would much rather hit the delete button than take a bunch of credit card / mortgage offers and catalogues to the recycling bin and spend that extra few moments sorting the mail.  I’m sure there’s a paper out there somewhere which confirms this hypothesis.