According to former TBWA\Chiat\Day Creative Director Ken Segall’s recently released book,Insanely Simple: The Obsession That Drives Apple’s Success, Steve Jobs was on the verge of launching an ad-supported version of the Mac OS 9 back in 1999.
From the patent description, it sounds much like the streaming video sites which disable playing the video until the ad has been viewed. The patent describes a method for OS to present ads to the user while disabling certain functions while the ad is being shown. “At the end of the advertisement, the operating system again enables the function(s),” reads the description. The ads being considered were both audio and visual, and would allow the user to obtain the OS either for free or at a discount.
Its important to remember that back in 1999, when the ad supported model was in its infancy, companies were launching all kinds of ad-supported products like free dial up internet that shows ads before you can connect etc.