It's a good idea and whoever came up with it got a patent. I know that because the number, "U.S. Patent No. 5,205,473" is printed in neat, black text right on the sleeve. (There's actually a second patent number as well.) The reason the sleeve on my coffee cup, and most other patented products, have patent numbers printed on them is something patent lawyers call "constructive notice."
Under the law, the public is deemed to have constructive notice that something is patented if that something has a patent number on it. The idea behind the law apparently is that if one sees a patent number, one has the ability to look that patent up, read it, and maybe even understand what it says. For the corrugated sleeve, it was simple enough for me--albeit a little geeky--to take a look at the '473 patent and understand how the sleeve works to make the heat from my latte more bearable.
But many times, the patents behind products marked with patent numbers are not as easily understandable. For example, the notice on Adobe Reader v. 8.1.2 lists over 60 different patents. I'm pretty sure that most of those patents are far more complex than the '473 patent. But, assuming the numbers are properly listed, the law says that I and the rest of the public are on notice of whatever it is those patents cover.
That means that if I got sued, I could be liable for things I did years before the lawsuit because I had notice. On the other hand, if there was no patent number listed on my coffee sleeve, the law may limit the patent owner's right to recover damages for stuff that occurred prior to the lawsuit. Simply put, constructive notice is a big deal.
That's why you see patent numbers listed all over the place. But is it fair? Reasonable minds could differ as to whether listing a slew of patents on a complex piece of technology (or even a couple of patents on a brown cardboard ring) really gives people notice of anything.
What's also debatable is why the law allows patent owners who don't have a commercial product--and therefore are not able to mark (after all it's hard to stick a patent number on a product you don't have)--to recover past damages without having provided actual notice of their patent. Obviously, the seeing-a-patent-number-and-looking-it-up theory for constructive notice can't apply in that situation.
So the question I ask, because admittedly I'm thankful for the protection offered by the corrugated-cardboard innovation of the '473 patent, is whether constructive notice is fair to the public and, if so, why is a patent owner who didn't mark worse off than one who doesn't have a commercial product to mark in the first place?