In VS2008, while in code-behind of your XAML page (.xaml.cs), pressing SHIFT-F7 will take you to the XAML Designer or the XAML code page, depending on which one had the latest focus. Pressing SHIFT-F7 again will take you from the XAML code page to the designer and vice versa.
While in the XAML designer or XAML code page, pressing F7 takes you to the related code behind page.
UPDATE.
Applicable for later versions (e.g. MS VS 2015 too).