As of this writing, the iPhone would not be enough anyways, as it is lacking any sort of to-do list feature (and it's not officially available in Canada, anyways). The lack of to-do lists on the iPhone is a great mystery, since you can add, edit and remove calendar events and contacts. On the surface, this seems simple enough to add.
Given Apple's way of doing things, I'm sure when we look back at this oversight, many will see this as foresight. Perhaps Apple is going to revamp to-do lists in iCal application, for instance. It's also possible that Apple hasn't been able to get to-do list synchronization working to their liking, and will address this in an update.
Anyways, I'm always taking three things with me. And, if I ever get a GPS unit, that will be four things. Add my keys and wallet, that's six things I'm scrounging around for before I can leave my home. I'd really like to see the iPod, phone, organizer and GPS combined into one. If there's some way to add the wallet in there, as well, I'd consider it. Of course, its more to lose in one shot, but if it's synced with my computer at home, then it's not nearly as disastrous as losing the only copy of your organizer.
It would be nice. And, it's so close to being possible I can almost taste it!