Not in any normal way.  Stephen Lebans has done some incredible things
with custom tooltips -- see http://www.lebans.com/tooltip.htm --  
including doing what you describe for a list box, but not (AFAIK) for a
combo box.  I assume the combo box is harder because its list isn't
always dropped down.  I'm not saying Stephen couldn't do it, mind you
...

The mouse over doesn't make a selection, so I believe the answer is no.
However, if you want the controltiptext to change when the user makes a
selection then in the afterupdate event use a select case routine to change
the controltip based on user selection:

Private Sub Combo0_AfterUpdate()
Select Case Combo0
Case "Red"
Me.Combo0.ControlTipText = "This is red"
Case "White"
Me.Combo0.ControlTipText = "This is White"
Case "Blue"
Me.Combo0.ControlTipText = "This is Blue"
Case "Yellow"
Me.Combo0.ControlTipText = "This is Yellow"
End Select
End Sub