I think generally speaking the word "major" is relative and also unnecessary. It only serves to confuse in this debate.
If your qualification is the primary function of the device as a gaming platform - personal computers from the very beginning and continuing all the way through modern versions have always been considered "gaming platforms" and none of those are strictly dedicated gaming devices.
iOS devices are essentially personal computers. Some make phonecalls as a primary function, but I'd venture to guess that those who own the iPhone variety of iOS devices probably put more hours into gaming/web browsing/using apps on it than they do using that phone feature.
Also, if the combination of the largest AAA developers/publishers (EA, Activision, Square, Namco, Konami, etc.) developing software product that is equivalent in graphical presentation, substance and content to what they're releasing on Sony and Nintendo's dedicated gaming devices, AND the independent game development community having a window into the marketplace to develop, distribute and profit off of games on a GREATER scale than they do with DS or PSP development (I doubt that companies like Rovio, Ngmoco, Chillingo etc. would have made the impact on the market and the very culture of portable gaming on PSP or DS even IF they had developed and produced games like Angry Birds, Rolando, Cut the Rope etc. for those systems instead of the iOS) doesn't stand to legitimize the platform as a place for real developers and real games - I'm not really sure what does.
Though, as I've stated in the past, I think that the marketplace is sorting this all out organically. With the natural saturation of iOS devices in the hands of more skeptical/doubting gamers there are more and more of these "epiphanies" every day via hands-on experience, and it won't take very long for a majority share of "core" gamers to come around to accept iOS as some type of equal, even if it's in a begrudging sense.