OFFICIAL PAGE: Drums Extractor
Drums Extractor, from the same developer as Vocal Extractor, is available as a free download for iOS. The app will take a mono or stereo source file and separate the drums (percussive elements). The developer is charging US$2.99 as an in-app purchase to unlock extraction and playback of content longer than 90 seconds. The results, even with mono source material, were quite impressive! The paid version of the app allows you to save your results as AAC or uncompressed WAV files. The app is available in the Apple App Store. Details are available at the link above.
Screenshot courtesy of Christopher Kissel