Outlook 2007 (and 2010) adds Next and Last buttons in the Reading pane and opened messages to make it easier to jump between the quoted messages. Not everyone likes this feature and if you are one of these people, you can disable message header shading.

In Outlook 2007, go to Tools, Options, Email Options, Shade message headers when reading mail.

In Outlook 2010, turn it off and on in Options, Mail – look for "Shade message headers when reading mail" in the Other section at the very bottom of the page. (Options is in the “backstage” area (Office icon button))