It's not exactly Warcraft vs. Half-Life, but Blizzard and Valve are crossing swords over the rights to use the term "DOTA" for an upcoming game.
As discovered by a poster on NeoGAF, Blizzard filed a notice of opposition with the United States Patent and Trademark Office last November over Valve's claim on a trademark for DOTA.
The DOTA name comes from a Warcraft III mod called Defense of the Ancients. In its opposition, Blizzard states that the DOTA name "for more than seven years has been used exclusively by Blizzard and its fan community, under license from Blizzard" and that Valve filed its trademark on the name "in order to confuse consumers as to Blizzard's sponsorship, affiliation, or endorsement of Valve's products."
According to the opposition, "By attempting to register the mark DOTA, Valve seeks to appropriate the more than seven years of goodwill that Blizzard has developed in the mark DOTA and in its Warcraft III computer game and take for itself a name that has come to signify the product of years of time and energy expended by Blizzard and by fans of Warcraft III."
In Valve's response, the company claims that Blizzard cannot claim exclusive rights to the name and that its own rights to Dota are senior to any claims made by Blizzard.
Both Blizzard and Valve are working on follow-ups to the original DOTA. The self-explanatory Blizzard DOTA is being built on the Starcraft II engine and will feature some of the developer's iconic characters as heroes in the game. That title was rebooted entirely last year and has no release date as of yet.
On the other hand, Valve's Dota 2 is being rushed to market with the company's plans for a year-long beta and full-featured launch scaled back so that it can be released "as soon as possible."