If you haven't obtained both of these trophies which require you to connect to online services...
Proud Owner - Take & Share a Photo.
Create Designer - Create & Share a Track.
The platinum for GT5 is now unobtainable because of the server closure on 30th May 2014. If you acquired both these trophies already however playing before the shutdown, it is still possible to platinum this game!
It is however a real time burner, especially with the endurance races in A-spec.