>>776
<Grid>
<StackPanel>

<ComboBox
x:Name="ComboBox1"
Width="100"
ToolTip="{Binding SelectedItem.ToolTip, ElementName=ComboBox1}">
<ComboBoxItem Content="This it tool tip for item 1" ToolTip="This it tool tip for item 1" />
<ComboBoxItem Content="This it tool tip for item 2" ToolTip="This it tool tip for item 2" />
</ComboBox>

</StackPanel>
</Grid>