I think disposables are the way to go for an introduction, as it forces them to focus on composition and the subject manner. Many people forget that the subject is the most important part of an image and how it is presented within the photo. And by using film, it will teach great patience and focus to create that image!
As far as purchasing disposables... I highly recommend staying away from cameras that aren't name brand (Fuji, Kodak...) or those who don't already have a solid reputation or guarantee attached to it. At work, I've developed many of the "wedding pack" cameras jnanian mentioned earlier and also many no-name branded cameras that actually had NO film negative inside the cartridge loaded into the camera or even film pieces that were poorly spliced together, often ruining many images. I assume this "company" acquired short ends of some sort and just slapped them together for their cameras. Just some precautions to be aware of.