Valve's DotA sequel will ditch its closed beta status and become 'free for everyone' later this summer, Erik Johnson told me during a visit to the developer earlier this year. We mentioned Valve's comments on the Dota 2 release date in PC Gamer issue 254 , and community-run Valve news site ValveTime have since confirmed the release window via an e-mail exchange with Valve.
Dota 2 is currently available via an early access purchase or - more likely - through one of the dozens of beta keys sitting in everyone's Steam Inventories. As such, full release might not seem like a big deal - but chances are this means a huge upsurge in interest. The game will open up prior to The International in August, the biggest tournament in the Dota calendar and a huge draw in and of itself. Dota 2 became the most-played game on Steam with hardly any marketing: with a multi-million dollar tournament and the publicity of full release behind it, the Dota population could be set to explode.
From my conversations at Valve I believe that the game's release will coincide with the launch of the next two heroes - Abaddon and Legion Commander, probably in that order. This may seem to clash with Valve's original assertion that the game would be released when it'd reached full parity with DotA 1, but it's not far off - there's only half a dozen heroes to go.
Valve have been inflating the prize pool for The International via the Compendium, an interactive book that acts as a reference document and theorycrafting minigame for the tournament. New rewards unlock as more Compendiums are bought, and the ultimate goal - still a way off, at the time of writing - is to give the community the ability to pick Dota 2's next hero. With the release window in mind, assume that this vote would take place during or after The International, and therefore that the pool of unreleased characters won't include Abaddon or Legion Commander. In other words, get ready for a lot of people to mash the 'Techies' button until their fingers bleed.