A couple of tools come to mind, though neither is a mind map. First, this sounds like the perfect application for Brainstorm, which is designed for exactly this purpose. Second option, which I've never used beyond a quick trial, is Sense by Silva Elm, but I think it does something similar.

http://brainstormsw.com

http://www.silvaelm.co.uk

And you might try Scrivener and just use the cork board view to start.

Steve Z.