Ah, morning—time to check the overnight e-mail that came in over the iPhone. Let's get to work!
Press the iPhone's power button. Tap the screen (that's screen tap #1) and drag the unlock slider. Press the Home button to get back to the main screen from whatever app I was using last on the iPhone. Tap (#2) the Mail icon. Oops, I left the iPhone reading a message the last time I was in Mail. Tap three times (#3, #4, #5) to get back to the list of accounts.
It's on this screen where you'll notice a big change from OS X's Mail program. In OS X's Mail application, the top-level folder is the global Inbox, and then within that folder, each of your accounts is listed separately, letting you easily view all of your new e-mail at once by just selecting the top level Inbox folder. On the iPhone, however, there is no global Inbox. Instead, the main Mail screen just shows a list of your accounts. You have to open each account to see the Inbox (and Trash and any other folders associated with that account).
Read my Macworld blog entry, iPhone, therefore iTap and tap and..., for the rest of the story...