Knuckles Dawson informed Xbox 360 Fanboy that backward compatible Xbox games are now showing up on Xbox 360 Gamercards after the Fall Update. The games do not appear on Xbox.com Gamercards, only on the 360 itself (under the Games Played tab in the Games blade.) The games are listed with the notice "0 of 0 Achievements." Also, only games you've played since the Fall Update will show up.
Xbox 360 Fanboy speculates as to why the old Xbox games are showing up on the list.