Valve have posted some details about DOTA 2's (previously revealed) free-to-play status, over on the the DOTA 2 blog. They're pretty upfront about two points: "Dota 2 will not be a pay-to-win game. All the items in the store are cosmetic, and don’t affect gameplay." But also: "All of the heroes will be available free of charge. We believe restricting player access to heroes could be destructive to game design, so it’s something we plan to avoid."
Perhaps more interesting still - and something I hadn't realised - DOTA 2 is already part of the Steam Workshop, meaning folks can submit items just as the have for Team Fortress 2: "If you’re a Dota 2 fan with some artistic skills, here’s your chance to get in on the ground floor of what’s sure to be a vibrant community for years to come, and be able to contribute directly to the game you’re already playing non-stop."