Well, this seems to have become a prevailing business model with iOS apps. There are plenty of ad-supported free or "Lite" versions of apps and then you pay to upgrade to the version with no ads. It seems to have become accepted better than we ever accepted adware on Macs or PCs (though they certainly existed).
I think it's silly at an OS level because using the OS is such a fundamental part of the computing experience, and also because if you're using OS X that means (usually) you've already purchased the Mac hardware. However, it might be a good way to have a "free trial" of the next OS. For example if I don't know if I want to upgrade to Mountain Lion, I could install an ad-supported free version and try it for a few months, and pay for the upgrade if I like it.
Now on the other hand, give me a free MacBook Air or iPad with the proviso that I have to watch ads before I can use it -- I'd probably go for that.
Or, as a poster above suggests, let me watch movies or TV shows on my Apple TV for free or at a discount, if I agree to watch ads. Still cheaper than cable.
Incidentally, I think the example of an ad for Epson ink wouldn't have worked out so well. If my Epson printer was running low on ink then of course I'm going to be buying Epson ink, as opposed to Canon or HP.