Anyone who publishes regularly, or even semi-regularly, will receive a steady flow of spam advertising new conferences and journals. The advertised publications vary widely in quality, and although some conferences are more academically reputable than others (with the frequently-spammed WMSCI notoriously on the low end), there is generally the presumption that the conferences themselves are legitimately organized. That is, whatever the expected quality of papers, one expects that the conference organizers are legitimately organizing an actual conference, and that payment of the registration fee will entitle the registrant to attend this conference, and report the conference publication on his/her CV.
This presumption of legitimacy (if not quality) is similarly true of spam advertising journals, special issues, calls for book chapters, and so on; however, it could easily be exploited. Especially since "getting a publication" motivates some academics to publish in lesser-known venues, particularly those who work in environments where promotion is due to quantity rather than quality of publications.
This came up recently, as I received an academic spam (similar to this one) offering to publish my five-year-old Ph.D. thesis as a book. Initially I found the offer interesting (whatever the quality of the publisher, it might make the thesis available to a wide audience via online bookstores), but in an unpleasant surprise, I discovered that the legitimacy of this offer is questionable. For one thing, they distressingly ask for your bank information, for the purposes of sending you "royalties" (fat chance, my thesis would probably sell less than 10 copies in total).
In spite of the controversy, there's enough room to believe that this publisher is probably legitimate, although I won't be taking up the offer. Nonetheless, it's not hard to imagine the following scam: harvest academic e-mail addresses, which are normally published along with papers; set up a bogus conference complete with fancy web page; spam-advertise the conference to the e-mail addresses harvested; accept everything; and once the authors register, take the money and run. Given the outrageously high registration fees for conferences these days (approaching a thousand dollars), this would be a remarkably lucrative scam if even a few people fell for it.
It's enough to make me cast a wary eye on my inbox full of new conference announcements.