I can't tell if you are thinking of using Lotus Notes as an individual or as part of an organization that uses it. If you are thinking of using it as an individual, I can't imagine why. There are much better solutions for single users.
We use Lotus Notes where I work, so I must use it. It will not work with the iPhone Mail app. There is a web-based solution called iNotes that will allow access to Lotus Notes Mail, both to read and answer. The Notes calendar is accessible to read only, and the only view is the current day. You can move one day at a time forward or backward in time, but you cannot add or delete appointments. You can see your contacts if they are on the Notes server, but you can't edit them. It does allow you to place calls by tapping on the phone number, though, or to send email to a contact.
I use this approach in a pinch, but I when I travel and need to be in real time contact with my email and calendar, I get the BlackBerry out of the drawer and take that. The iPhone solution is too limited. Unfortunately.
I second the endorsement of MobileMe. It has become more and more valuable, the more I use it.