Rolling releases are not new. Gentoo was one of the first, if not the actual, that was considered a rolling release. Later rPath, then PCLOS came along and Sabayon followed. Arch joined the fray and openSUSE began Tumbleweed. But more projects are kicking around the idea these days and some have even done it.
Rolling release are those systems which are updated in smaller increments over time usually from within the system with a software management client as opposed to the more traditional installation of a new system every so often. The advantages to the user is obvious, but the developer has his reasons as well. That's why more and more projects are implementing the rolling release model, or are at least talking about it. Here are two very recent examples.
rest here