Now Gmail can check for the mail you receive at your other email accounts. You can retrieve your mail (new and old) from up to five other email accounts and have them all in Gmail. Then you can even create a customized 'From:' address, which lets you send messages from Gmail, but have them look like they were sent from another one of your email accounts. Please note that you can only retrieve mail from accounts that have POP3 access enabled. Learn more
This feature is currently only enabled for a limited number of users. We're working on making it more available soon. Look for it in the 'Accounts' tab in Settings.