I'm a big fan of note taking apps. All the apps mentioned are great but I think at the end of the day nothing beats Notational Velocity. It is simple and comes up fast. I have it tied to a keyboard shortcut so I can quickly bring it up jot down what i need and get out. Since it stores its data in standard rtf files it is not tying you to some dB format that is not easily exportable later on. If you send the author an email I know there is a universal binary version available although even the current PPC version works great in emulation since the app is so small and simple.

http://notational.net/

I'm also a fan of mori but at the end of the day I think all these outliners make me spend too much time thinking about where I should put something. I prefer a nice long list of items that I can search and what is important is having keywords associated with the item such that when I do that search in the future I will find it. YoJimbo basically works this way but the next time you are on the phone and somebody wants to give you a bit of info, see how long it takes to get YoJimbo up and running before you can actually type in the piece of info you were just given. Mori will come up quickly but then you need to look at this big beautiful tree you have built and decide where it belongs. In notational you just bring it up and start typing.

My 2 cents