If you don't know what Defense of the Ancients is...well, you're probably not reading this post. The super popular custom scenario for Warcraft III got enough attention for Valve to want to do it themselves. Or at least that has been the running rumor.
There was that one tweet from a voice actor that said he had fun "recording for DotA" while also naming Valve. Before that Valve had hired DotA dev Icefrog. And now there's a trademark filed by Valve for "DOTA."
I don't know what other kind of proof you need.
Did we need another DotA-style game? We have Heroes of Newerth and League of Legends. Oh, and the original. Then again, this is Valve we're talking about. They could go crazy and do something fresh/new with the formula.