Like the Treo, the iPhone has separate applications for SMS and email; and like the Treo, the SMS application displays messages in a threaded conversation like an instant message.
The list view of both application displays messages with the sender in bold and the first 10 words or so from the message below. Unread messages have a blue dot next to them. If you click on a message, that is where the two applications differ.
The SMS application displays incoming and outgoing texts in speech bubbles. Your messages are on the right and in a green bubble, senders' messages are on the left in a white bubble. The iPhone makes every attempt to keep all messages from a single sender in one thread.
The email application is more like a traditional inbox. Messages are listed from newest to oldest and selecting a message displays just that one. Plain text messages are displayed instantly, however HTML formatted messages take longer to display - up to about 5 seconds depending on how complex their formatting is.
Viewing an HTML message works exactly like viewing a web page. You can tap or pinch to zoom in and out. Scrolling also produces the same strange behavior in both applications. The part of the page displayed on screen is rendered first, so if you quickly scroll down (or over, or up) you'll sometimes see the checkerboard empty page while the iPhone works to draw what should be in that area.
The mail application supports multiple accounts, with a separate inbox for each one.
Although many people complain about the lack of Exchange support in the mail client, it's clear that the iPhone is not meant as a corporate mail device. It does not have any way to deal with a number of messages in bulk nor does it really have any means of triaging messages like road warriors do.