I wanted to share a further improvement on this that I finally discovered. One of the remaining issues was that setting the ToolTipText= "" does not turn off the displayed tooltip immediately as it would normally when you move off of an item. So to get the ToolTip display to go away as soon as the cursor is moved off of the grid column, you can do the following. (Good ole INKEY() to the rescue again). I couldn't find any other way to make this work without using INKEY().

* Add this to the Column.MouseLeave event
this.parent.ToolTipText= ""
thisform.ShowTips= .f.  && now turn off currently displayed tip
INKEY(.1)
thisform.ShowTips= .t.

* Add this to the Column.MouseEnter event, (as Andreas provided).
this.parent.ToolTipText= this.ToolTipText

-Mark