I'm using Mail.app (on Mac OS X 10.6) and when I send an email to users on PC Outlook, with an attached image, they get the email as an image embedded into the body, not as an attachement.
I even tried clicking "view as icon" before sending the attachment from Macmail, but that made no difference. I also tried this myself, sending from Mail.app over to my PC's Outlook, and I do get that same problem. In Outlook the image is not coming through as an attachment, but as an image embedded into the body of the email.
The reason this is an issue primarily is because the user is then unable to click "save as" and has to actually copy and paste it into some other program, which means the file is converted from jpg or png to the bmp format. But beyond that, most of my recipients don't even know how to copy and paste it into another program to save it that way anyway. They need the "save attachment as" functionality.