On the official twitter page for Jon St. John (the voice actor of Duke Nukem) he tweets...

Had a great time in Seattle last week recording for DotA. The guys at ValveSoftware are awesome!

Source: http://twitter.com/JSJakaDukeNukem/status/20491803584

It was also stated on IceFrog's blog that he was leading a team at Valve. (IceFrog is the lead developer of DotA)

Source: http://www.playdota.com/forums/blogs/icefrog/264/great-news--dota-fans/

The plot thickens...

The tweet of Jon St. John has since been removed, probably by Valve telling him to remove it since Valve love to keep things a secret.

But new evidence has surfaced with Valve officially copyrighting the word/phrase DOTA.

Source: http://tarr.uspto.gov/servlet/tarr?regser=serial&entry=85102245

So looks very likely Valve are releasing DotA as their own game, going in the success of Alien Swarm.