I have the following lines in my pop-up form code:
With Forms!frmName.FieldName
.Undo
.Requery
.Value = Me.NewValue
End With
The ".Undo" removes whatever you originally typed into the field that triggered the NotInList Event. The ".Requery", as pointed out by sstreet, requeries the combo box, and is all that you really need. The next line sets the value of the field in the orignal form equal to the value that you've just created for the new record in your pop-up form.