I believe that only users who have voluntarily installed the relevant IE update (which is not required) will see this.
I think what I'm going to do is inform my clients of the problem, maybe send them these links
http://www.macromedia.com/devnet/activecontent/#captivate_before
http://www.macromedia.com/devnet/activecontent/articles/javascript_solution2.html
to let them see what happens, and also show that it's not just a problem with their site but is going to be a problem with any site that uses any activeX content.
As for who pays for it, I think the best solution is to give them a discounted rate to implement the workaround. It's not our fault either, after all, and sites do need updated for lots of reasons over the years. Customers can't really expect their site to work forever, no matter what changes happen at the browser level. It's just part and parcel of having a website. Updates need to be paid for.
Take the example of unleaded fuel. When lead was removed from petrol/gas (in most countries), older engines needed to be modified. Although car manufacturers had to start producing engines that didn't run on leaded fuel, they didn't pay for the required changes to older ones, and neither did the oil companies or the service stations. The end user did. Tough poop.
Personally, I think it's the fault of Eolas and the University of California for being bloody-minded enough to force the change to IE after getting over $500m for doing next to nothing, and Microsoft for not checking the patents properly in the first place. Maybe we should start a campaign to send the parties involved a bill for the time and inconvenience they caused. They seem to have more money than me...
Incidentally, there may need to be changes to other browsers too, if Eolas is going to be greedy enough to try to sue them as well.