I discovered this on my current project. Bizaar - web part pages never render the left navigation. This looks crap, because you flick from publishing pages that display the left nav to web part pages that don't.
I hunted around to find a fix and here it is...
The Master page contains the following two Content Placeholders, which are instructed by the web part pages not to load.
<asp:contentplaceholder id="PlaceHolderLeftNavBar" runat="server"> "CODE" </asp:ContentPlaceHolder>
<asp:contentplaceholder id="PlaceHolderNavSpacer" runat="server">"CODE"</asp:ContentPlaceholder>
Basically within these two content placeholders you move each 'CODE' block outside each ContentPlaceHolder tag.
NOTE:DO NOT delete the Content Placeholders, as they are still needed to render (load) the page. The page will error if they are deleted.