When I spoke to the TWA rep on the phone, she said that those with no recent mileage activity on TWA would be "added to the database" later in the year... implying a sort of customer ranking. Since they are also perfectly willing to combine miles instantly in the case of a mileage award on AA, there's no real concern about when the miles will be added ... now or later. Unless you are just one of those anal retentive folk (*raises hand*).