Valve formally announced Source 2 in March, long after we first saw scraps of it in Dota 2 [official site], but when will get to properly play games in it? Soon, is the answer, and it looks like the first will indeed be Dota 2.
The company last night announced Dota 2 Reborn, a revamp of the free-to-play lords management game switching it to a new engine (they don't name Source 2, but c'mon) with a new UI, revamped spectating, what I imagine is support for custom game modes, new ways to ease new players in, and more. This version will launch into open beta next week, then properly launch after August's big tournament The International.
Valve are doing their usual multi-phase reveal, with the first part focused on the new UI, spectating, and teachy bits. The new UI looks way better than the current trash heap, and that's about all I have to say on that. See for yourself on the site.
As for newbies, Valve are planning a system of 'Guided Bot Matches', where an AI director and tutorials will walk new folk through games. Valve say, "Striking a careful balance between providing enough information to be interesting, and not so much as to be overwhelming, these tutorials should be fun and educational for any beginning player, and give them a taste of what it's like to play a full game of Dota."
Early next week Valve will have more to say about custom games, which are basically mods. Dota 2 does support them right now, but it's alpha, you need to download an add-on, and they're hidden in a corner. I imagine they'll be dragged out into the spotlight with Reborn. Later next week, they'll talk about the new engine (ooh!) and finally launch the beta for all to try.
I am pret-ty excited for this.