One of the few bad things about the mail client in the iPhone is that, unlike Safari, you can not tilt the phone to get widescreen mode.
Since the text is jus a bit larger in widescreen by default this would be exceptional good for reading my many html mail. No mostly I have to first zoom some to get a proper text-size.
Hopefully this is a upcoming feature in the early software updates for iPhone.
Please Apple, please ;)
Could be better: