I’ve known about this for about a week (thanks, various Twitter and email tips). I wouldn’t be surprised if offline support showed up in iOS 6, too. It’s a glaring feature omission, and it’s far more important on iOS than on Macs.
Like today’s Reading List, it will certainly prevent some people from buying Instapaper, but might encourage more customers to seek out a more robust app to solve this problem.
Since Reading List’s release in Lion and iOS 5 last year, a lot of people have asked me about its effect on Instapaper. It’s effectively impossible to correlate long-term App Store sales trends to their causes, so I really can’t be sure. So far, Instapaper’s sales are still strong despite far more (and far stronger, and all free) competition this year. Maybe sales would have been stronger without Reading List, but I doubt it.
I predict approximately the same effect — nothing obvious — when Mountain Lion and presumably iOS add offline saving to Reading List this year.