If you use Google Apps and Outlook 2003 or later (this also works for a multitude of other desktop mail programs), simply download the newly-released Google E-Mail Uploader. It's not clear if this works for regular Gmail accounts, but it definitely works for Google Apps accounts.
If you use Gmail or Google Apps, simply turn on IMAP access, configure your desktop client appropriately to access your account via IMAP, and drag and drop your e-mail into the Google IMAP folders.
Both of these methods preserve all of the original e-mail information, including the correct dates.
Thanks to everyone that worked with me in the past to come up with workarounds to make the upload of e-mail to Gmail easier; it's now so easy that it fits into this small of a post.