These two collections seam to be very similar, Balmain SS//2011 [all lefts] and Isabel Marant SS//2011 [all rights]. It becomes even more clear when seen on the same models: Sasha Pivovarova [all aboves] and Anabela Belikova [all belows].
The reason that the collections appear similar can be found in the collaboration of both fashion houses with the same stylist: Emmanuelle Alt. Some rumors are saying that from next season on she will no longer work as a styling consultant for both brands, due to her new position as editor in chief at Vogue France... This could be a great opportunity for Balmain and Isabel Marant to get straight their identity.