Windows XP and Vista only: Google releases an open source email uploader that moves existing email and contacts in your desktop client into your Google Apps Gmail account. (That's Google Apps Gmail account, not vanilla Gmail.) Download and install the Google Email Uploader, and choose mailboxes from Microsoft Outlook, Outlook Express, and/or Thunderbird to start uploading. After the jump, screenies from the process.
The Google Email Uploader is a free download for Windows XP and Vista.