For all of who use Randy Drisgill’s starter masters, it won’t help if you go to Site Actions TreView and just enable it, because the DelegateControl is in a hidden panel. To really restore the original treeview you have to copy this xml from v4.master:

<Sharepoint:UIVersionedContent runat="server" UIVersion="4">
	<ContentTemplate>
		<Sharepoint:SPNavigationManager
		id="TreeViewNavigationManagerV4"
		runat="server"
		ContainedControl="TreeView"
		CssClass="s4-treeView"
		>
		  <SharePoint:SPLinkButton runat="server" 
			NavigateUrl="~site/\_layouts/viewlsts.aspx" 
			id="idNavLinkSiteHierarchyV4" 
			Text="<%$Resources:wss,treeview\_header%>" 
			accesskey="<%$Resources:wss,quiklnch\_allcontent\_AK%>" 
			CssClass="s4-qlheader" />
			  <div class="ms-treeviewouter">
				<SharePoint:DelegateControl runat="server" ControlId="TreeViewAndDataSource">
				  <Template\_Controls>
					<SharePoint:SPHierarchyDataSourceControl
					 runat="server"
					 id="TreeViewDataSourceV4"
					 RootContextObject="Web"
					 IncludeDiscussionFolders="true"
					/>
					<SharePoint:SPRememberScroll runat="server" 
						id="TreeViewRememberScrollV4" 
						onscroll="javascript:_spRecordScrollPositions(this);" 
						style="overflow: auto;height: 400px;width: 155px; ">
					  <Sharepoint:SPTreeView
						id="WebTreeViewV4"
						runat="server"
						ShowLines="false"
						DataSourceId="TreeViewDataSourceV4"
						ExpandDepth="0"
						SelectedNodeStyle-CssClass="ms-tvselected"
						NodeStyle-CssClass="ms-navitem"
						SkipLinkText=""
						NodeIndent="12"
						ExpandImageUrl="/_layouts/images/tvclosed.png"
						ExpandImageUrlRtl="/_layouts/images/tvclosedrtl.png"
						CollapseImageUrl="/_layouts/images/tvopen.png"
						CollapseImageUrlRtl="/_layouts/images/tvopenrtl.png"
						NoExpandImageUrl="/_layouts/images/tvblank.gif"
					  >
					  </Sharepoint:SPTreeView>
					</Sharepoint:SPRememberScroll>
				  </Template_Controls>
				</SharePoint:DelegateControl>
			  </div>
		</Sharepoint:SPNavigationManager>
	</ContentTemplate>
</SharePoint:UIVersionedContent>

and paste it before: