Nick, think you meant to include the second argument as well:
Sub CountConstants()
Dim lConstantCount As Long
On Error Resume Next
lConstantCount = Selection.SpecialCells( _
xlCellTypeConstants,xlTextValues).Count
On Error goto 0
MsgBox "There are " & lConstantCount _
& " Cells with constants in the
selection"
End Sub
Just in case there are constant numbers in the selection as well.