I've mentioned some options for iPhone-based mind mapping before, but I'm pretty excited to let everyone know about the release of MindMeister for iPhone (iTunes Link). MindMeister is a web-based mind mapping application with excellent import and export features for most major formats.
They've been focused on collaboration from the beginning, allowing realtime editing of mind maps by multiple participants, and including Skype integration. The iPhone version allows you to download your maps from your account (or start from scratch) and sync changes back to your maps, share maps, and export as MindManager, Freemind, Word, PDF or image files.
MindMeister for iPhone (and iPod touch) is an evolution of the MindMaker application, which was acquired by MindMeister. While the online version of MindMeister allows for a range of icons, images and metadata on each node, the iPhone version currently lacks a lot of this functionality. However, it provides a fast, simple way to edit the text of nodes, adding to existing maps or creating new ones on-the-go. The navigation is an intuitive pan-and-zoom, tap-to-edit interface which is pretty standard among iPhone mind-mapping apps. It also incorporates the GeistesBlitz feature of MindMeister, which allows you to instantly add ideas to a default map, or create a basic map from scratch, through email or a variety of other methods (there's a REST API available if you feel like adding your own points of access).
As an aside, I'd like to point out that the online version has some cool tricks. For example, you can import a tab-indented outline from a text file and turn it into a map automatically (just go to "Import Mindmap" and select a text file). Also, if you're editing a map, select a node and then click the image or note button ... not the dropdown menu, just click the button. It will do a quick search of Google Images or Wikipedia, respectively, and insert the results. Don't like the image it found for your text? Click again. Try it, it's fun.
While I'm not completely sold on iPhone mind-mapping (paper is pretty fast), the syncing, sharing, rapid-entry features of MindMeister make it my top pick. It's not just about device portability, it's about data portability; with MindMeister, my maps can end up in just about any format I want them to, and I can share them directly and get feedback instantly. MindMeister for iPhone is currently $6.99 in the App Store. If you're a MindMaker user, it's a free upgrade. The MindMeister.com online service has plans ranging from free to $6/month, with a $15/year academic option.