I had a very similar idea a few weeks ago, since I usually track to-do lists in a text editor anyway. But my idea (and what I would recommend) is no modes. There's no reason you can't have the normal UI act like a giant textarea. The really important part is supporting tab/shift-tab while doing this.
The (non-free) Mac desktop program OmniOutliner (http://www.omnigroup.com/products/omnioutliner/) also handles outlines like this. I use it for taking notes on a lot of things. It has its flaws, but it’s decently powerful – I use the custom text styles a lot for marking up code and file paths in my text.
I can also vouch for OmniOutliner, though I personally don't use it for task management. I use Notational Velocity (http://notational.net/) for taking reference notes, and for task management I email myself action items and tag them using the GTD system. Gmail's amazing tagging and filtering system and its abundance of single-key keyboard shortcuts make it better than any other system I've tried. I don't even miss being able to edit items; I've gotten used to either forwarding or making a new one, and it's a good tradeoff. I'll write an article about it someday.
For nearly everything else I'll default to OmniOutliner, and I usually have at least three or four windows of it open. I've never found any outlining software that comes close. Favorite features: Rarely lacks a keyboard shortcut I want it to have, has audio recording built in (I'd suggest an external mic for it to be useful though), and exports to plain text, HTML, RTF, and Word.
