I'm making this thread because I'm betting a lot of people don't really know about it. So far we've seen Nintendo show the image attachment icon for Miiverse chat messages, but the Wii also had image attachment, as does PSN messaging. If anybody is like me, then they would be unsure if this indicated a universal screenshot tool, or simply provide an attachment option for game-specific screenshotting. Again, like Wii and PlayStation 3 games.
Going by the following quote from the aforementioned Iwata Asks, the Wii U does indeed have OS level screenshotting:
For some who expected this based on the previous Miiverse images, this thread might seem pointless. But I personally think Nintendo's communication of features is dreadful, and believe this is the first time they've outright confirmed OS level screenshotting for games.
Kurisu: I want people to try Miiverse just by itself, and also try posting to Miiverse from games. If you spot a cool scene, you can post a screenshot by pressing the HOME Button. I truly hope people will actively participate in the communities tied to particular games.
Iwata: After you press the HOME Button, you can go from the HOME Button Menu to Miiverse. The game screen that was displayed at the time will be saved as a screenshot that you can post, right?
Kurisu: Yes. There's a spoiler button, too. We've designed it so that people who don't want to see spoilers won't have to.
Iwata: There's also a way to report something that you feel is a spoiler so it should be hidden.
Kurisu: Yeah. I hope people will make use of such features and enjoy aspects of Miiverse that connect to the games.
And because I think that's cool; thread.