The consensus seems to be this:
If under iOS 6, a USB DAC worked with an iPad using an appropriate set of adapters -- either 30-pin CCK, Lightning-to-USB adapter, or either combined with particular powered or unpowered USB hub -- then with some limited exceptions, it now works with an iPhone and (presumably) an iPod Touch under iOS 7. Basically, the capability that was limited to iPads now applies to iPhones and iPod Touches that can load the latest iOS.
The need for MFi is still there for iPhones/iPods when using a standard 30-pin or Lightning cable. Also, certain DACs still need a powered hub or some such hub "trickery" to work. One example is the Fiio E17, which I've gotten to work on both an iPhone 4 and an iPhone 5c running iOS 7 with the CCK or Lightning-to-USB and a particular hub I have. And some DAC manufacturers have reduced their current draw to allow the CCK to work without a powered hub, like HRT, which now works with iPhones, iPads, and (presumably) iPod Touches, all on the latest OS.