Upper left of the main screen after launching the mail app should show all email accounts that are setup. To switch between/among them (for example, work email and personal email addresses should be visible). Just tap on the email that you want to check, then tap on the Inbox to see all emails for that account. Tap get back to all accounts, tap in the upper left on the email address, then tap on Accounts to get back to the main page that display all email addresses. iOS 4 should be bringing a unified inbox this fall.