When I place a control on a tabpage in Silverlight the control is placed ~10 pixels down and ~10 pixels right. For example, the following xaml:
<System_Windows_Controls:TabControl x:Name=TabControlMain Canvas.Left='0' Canvas.Top='75' Width='800' Height='525' Background='Red' HorizontalContentAlignment='Left' VerticalContentAlignment='Top' Padding='0' Margin='0'> <System_Windows_Controls:TabItem Header='Test' VerticalContentAlignment='Top' BorderThickness='0' Margin='0' Padding='0' HorizontalContentAlignment='Left'> <ContentControl> <Grid Width='400' Height='200' Background='White'/> </ContentControl> </System_Windows_Controls:TabItem> </System_Windows_Controls:TabControl>
will produce:

How do I position the content at 0,0?
Look at the control template, it has a margin of that size. Use blend to modify the a copy of the tab control’s template.