A terrible oversight on the iPhone is the lack of a usable task list. Between the useless calendar and no task list the iPhone - as a organizer - is less worthless. I miss, quite frankly, the Treo for that. There is no way to synchronize (that I've found yet) between the iPhone and Outlook, for example.
I've been playing with web-based task list sites. Most that I've found are simply too slow when loading on Edge, and difficult to use in the browser even after loaded. One that I've used and like is from BaseCamp, and though it loads nicely on the iPhone it is quite slow and not formatted for the screen.
One that I've found that us very nice and productive, and has been designed specifically for the iPhone, is VitaList. They have a special URL for the iPhone: iphone.vitalist.com .
The "system" that it follows is the found in the Getting Things Done (GTD) book - which I have but still haven't gotten around to actually reading (it's one of the oldest ignored items my task list). But the approach is straightforward and doesn't require following some complicated method of personal organization. When used in conjunction with the desktop version of the site it is quite powerful. Projects and contexts can be easily setup and used on the iPhone.