Heavy Ephemera has released wiresq, a turing complete sequencer for generating musical phrases.
It uses wireworld, a cellular automata that allows for the construction of digital logic circuits to generate audio triggers. You can construct logic gates, memory circuits, calculators and more. You may be familiar with similar applications that use the game of life, but in this CA is much easier to construct stable repeating patterns and logic gates.
It also includes a simple synthesizer with an agressive filter. Enough to sketch out your ideas, but not too much to get bogged down in.
wiresq for iOS 3.1 or higher is available to purchase from the iTunes App Store for $1.99 USD.
More information: Heavy Ephemera (website no longer available)