Flickr debuts Collections
As you can see from the screencap below, there are a couple of quirks. First, collections can only be nested 5 deep. for most people, this shouldn't be an issue, but some power users may hit that limit fairly quickly. Second, any collection can be composed of sets or other collections, but not both. That means if you have a collection and you want to create a new collection with that collection plus another set, you have to put the set into a collection first. There doesn't seem to be anything to stop you from creating a collection with a single set, so it's no big deal, but it is an extra bit of hassle, and it potentially costs you a layer of nested collections.
Finally, drag and drop editing of sets and mosaics doesn't seem to have the kinks worked out in Safari yet. When I clicked on the icons to arrange the mosaic on my test collection, the icons floated around the window nowhere near the mouse pointer. And the finished collection never showed up, even though everything looked fine. Firefox seems just fine, though.