Presumably when Microsoft starts giving away Word and makes Visual Studio a compelling platform for cross-platform development?
In all honesty, given the iPad's share of the tablet market, making unrestricted, high-quality, cross-platform interactive content development tools available to iOS developers at no cost might provoke antitrust claims from the likes of Adobe. But Adobe isn't likely to sell expensive digital publishing software to authors of free books, and mainstream publishers are unlikely to commit to "iPad exclusive" titles to avoid a few thousand dollars in software licenses.
In all honesty, given the iPad's share of the tablet market, making unrestricted, high-quality, cross-platform interactive content development tools available to iOS developers at no cost might provoke antitrust claims from the likes of Adobe. But Adobe isn't likely to sell expensive digital publishing software to authors of free books, and mainstream publishers are unlikely to commit to "iPad exclusive" titles to avoid a few thousand dollars in software licenses.