For notes on my iPhone, SimpleNote has been awesome.
It doesn't have the ability to categorize notes, but it is fast and simple. It also includes TextExpander integration on the iPhone/iPod touch side which is very helpful for me. With SimpleNote you can sync to their web service which will allow you to view your notes on your Mac and it syncs the notes locally on your iPhone/iPod touch which would you allow to view your notes offline.
I have WriteRoom as well and it is very similar to SimpleNote. WriteRoom is very good app on the iPhone.
I actually use both for different functions. SimpleNote is my quick notes, daily notes, jot down something quickly, idea, and miscellaneous bucket. WriteRoom is my writing/creative project development bucket.
To make a Moleskine analogy, SimpleNote would be my pocket journal for daily, miscellaneous notes, and scribblings. WriteRoom would be a larger Moleskine journal for more developed writing and working out the writing itself (i.e. taking the ideas from my scribblings and ironing it out in the bigger journal).
As for using OF for a notes function, you could assign an "On Hold" context for notes. You could have several of these for different categories of notes. This might work for what you are looking for...
However, IMO the headache created by having my notes and tasks intermingled in one app or database would bother me (insert OCD joke or reference here). I like the hard lines between different types of data or their meaning to me. Even when one app could do everything pretty well, I prefer to have the individual apps that do that one thing great and are streamlined for that function. SimpleNote or WriteRoom would be more than worth it to me for their purchase price ($1.99 and $4.99, respectively and currently). Their purchase price includes the syncing service to allow you to view your notes on your Mac, or any computer that get online for that matter.
Other options would be NoteLife and DEVONthink (when their iPhone app ships) for use on the iPhone/iPod touch as mentioned by others here. I haven't used NoteLife before so I can't speak on its merits.
It will come down to your personal preference on how you will use the app and how it will fit into your flow. It doesn't matter how powerful or great the app is if you won't use it. That would be my concern with using OF as a notes app, it was not designed with that use in mind. So, the friction to creating or viewing "notes" in OF would be high and therefore a user would be less likely to use it in that way.
Just some thoughts, good luck in finding what you are seeking.