Kadri, there is one new cloud parameter that allows the samples to skip over large volumes of empty space, but I didn't use it here. With this render, the main difference between how it renders in 2.0 and 2.1 is that I ray-traced the pixels, rather than breaking the background sphere into micropolygons and shading those. This is done using a new option called "ray trace everything". For most landscape scenes you still need to use the old rendering mode, but it wouldn't have made a huge difference on this scene anyway.
So basically these clouds are possible in 2.0. I'll post an example scene soon.
Apart from that, not much has changed in the clouds for 2.1. The biggest thing we've been working on is "ray trace objects", which improves quality and speed of your imported objects and vegetation, especially populations. It also means we avoid some of the bugs in the old population rendering engine which we think may have caused many of the render errors people were seeing.