Ruches are conquering every designer. During the last seasons almost every brand decided to use this "decoration" in some different ways. For example, also Simons for Sander uses ruches on maxi skirts; the most minimalistic brand uses ruches. This is one of the things that I would not have ever imagined.
However, we see Valentino, Marchesa, McQueen, Ricci...everywhere an explosion of ruches. McQueen is such an exageration of ruches that only Marchesa 's gown has more of them.
I don't know if this is a trend that will end with this season or will continue...Speaking about f/w 2011 this trend is not so spread even if some brands use ruches (but, as I told, some maisons like Valentino and so on use ruches everytime.
Pilati for YSL is the only one who uses contrasting ruches. In fact, on the neck uses pink that contrast with blue and on the hem there is a little ruching underlined by some white pieces of fabric.