I discovered NodeBeat [iTunes link] by accident after seeing it demoed on Youtube.
Based on the idea of note generators and receptors, the app offers a quick and easy way to make pleasing sounding ambient audio on the iPad.
My video below gives you an idea what it's about.
Actually, it was while playing around with this app I was reminded of an old Jean Michel Jarre track, Arpegiator, from the Concerts in China album. See what you think [iTunes link].