I just discovered this app - iTunes blurb below:
A Novel Idea is the premier tool for plotting your story and recording bursts of inspiration. Use its simple interface to create your characters, locations, scenes, and novels and then link them together to create your story's plot. Use the Idea feature to quickly jot down your creative sparks and link them to your story elements.
Downloaded for a play. Looks good - pity no timeline though. Another good thought capturer I was using was Werdsmith but I think Novel Idea has more features.
Paid iPad app ($5) which provides the index card functionality of Scrivener and integrates with it. So you can plot and scheme on your iPad, and then synch it with your larger project in Scrivener.