Try defining the component in-line instead. You also don't want to use
the ToolTip component because you still need to display the header
text. Instead, just use a Label component and set the toolTip string
for the label like so...
<mx:DataGridColumn
width="26"
textAlign="center"
editable="false">
<mx:headerRenderer>
<mx:Component>
<mx:Label text="M" toolTip="My Header Tooltip"/>
</mx:Component>
</mx:headerRenderer>
<mx:itemRenderer>
<mx:Component>
<mx:HBox horizontalAlign="center"
click="outerDocument.moveToActive();">
<mx:Image height="16" width="16"
source="@Embed('/images/backwardicon.png')"
toolTip="Click to make 'Active'" >
</mx:Image>
</mx:HBox>
</mx:Component>
</mx:itemRenderer>
</mx:DataGridColumn>
