I put this markup on a new page and replace each RadMultiPage content with just a label ... it works fine. I thought maybe the code-behind was accessing some part of this control, so I changed the name of all IDs expecting to get a compilation error. However, as expected, the code-behind is not accessing any part of this control.
<asp:Panel ID="pnlTabStrip" runat="server" Visible="true">
<telerik:RadTabStrip ID="rtsContact" runat="server" SelectedIndex="0" MultiPageID="rmpContact" ReorderTabsOnSelect="true">
<Tabs>
<telerik:RadTab Text="Contact Mechanisms" />
<telerik:RadTab Text="Relationships"/>
<telerik:RadTab Text="Notes" />
</Tabs>
</telerik:RadTabStrip>
<telerik:RadMultiPage ID="rmpContact" runat="server" SelectedIndex="0">
<telerik:RadPageView ID="rpvMechanism" runat="server">
<uc1:WucContactMechanism ID="wucContactMechanism" runat="server" />
</telerik:RadPageView>
<telerik:RadPageView ID="rpvRelationships" runat="server">
<uc2:WucAssignedParties ID="wucAssignedParties" runat="server" />
</telerik:RadPageView>
<telerik:RadPageView ID="rpvNotes" runat="server">
<telerik:RadTextBox ID="txtNotes" Runat="server" Rows="15" TextMode="MultiLine" Width="650px" TabIndex="90" BorderColor="LightGray" BorderWidth="1" EnableEmbeddedSkins="false" Skin="IBMS" />
</telerik:RadPageView>
</telerik:RadMultiPage>
</asp:Panel>