There's no exposed filesystem within iOS, which can be frustrating, but there are pretty easy workarounds. For list management, I currently use Google Tasks on the desktop, SSI gTasks on my Nexus S, and GeeTasks on the iPad.
Originally Posted by pxt
For checklists and notes, I use Simplenote, which is a plain text repository with clients for the web and iOS. For paid accounts (which I have), Simplenote syncs to Dropbox. It also has third party clients for Windows and OS X, as well as extensions for Firefox and Chrome. I could probably organize everything--next actions, checklists and memos--within Simplenote exclusively, but have stuck to Google Tasks out of habit.
You could also use ToDo.txt, Gina Trapani's plain text list manager that works on everything from Android to the Bash shell.
I use Evernote, but only for capturing recipes or gadgets that I might want to explore later--items that would be too frivolous and unwieldy to review weekly in a Someday/Maybe list.