Pablo,
The grid is created before the form's INit runs (the Form's Init is the LAST thing that runs sduring the creation of a form). If you opne the table in teh Form's Init then it is not open when teh grid is created and its Init runs so you get the error, alias is not found because the alias doesn't exist yet.
By waiting to set teh column's controlsource until after the table has been open you don't get the error because the alias does exist.