Martin, think I haven't formally introduce myself, but I do work in publishing myself

for quite a reknown business books publisher. I'm aware of the different format of ebooks and the one you refer to as being the Windows version is Microsoft Reader format and as you correcntly stated, no Mac version is available. I'm aware that most ebooks are on adobe PDF format which is accessible to almost all computer platforms inc PDAs and Smartphones. Theres also another format called MobiPocket.
Like I said, PDF's CAN be more than password protected, but it would cost in the region of $5000 and some web server configuration and development before it is possible. I think it had to be PDF version 6 or 7 though which I can see may limit the PDF audience but all they need to do is download the latest version of Adobe Acrobat Reader.
Have a look at
http://www.fileopen.com/publisher.html which would encrypt the PDF ebooks and you can set up options like how long they can view it, if they can print it, copy it, forward it, edit it etc. Its an option I personally looked into and know works.
Don't take my word for it, there are ebook shops already doing it. Have a look at
http://www.ebookstore.tandf.co.uk/html/mor...ookid=536877088 (BTW, this isn't the company I work for, just a competitor) so I know its possible.
Only thing I would say is its not cheap and after doing my research there are no cheap ways of doing this (well there are but limited in functionality).