It caused a bit of controversy when Valve went ahead and register Dota (Defence of the Ancients) as they prepared to ready Ice Frog's next take on the genre. And Blizzard's whose game Warcraft III was the game the mod Dota first appeared on, have now challenged Valve's trademark application.
"Valve has no right to the registration it seeks. If such registration is issued, it not only will damage Blizzard, but also the legions of Blizzard fans that have worked for years with Blizzard and its products, including by causing consumers to falsely believe that Valve's products are affiliated, sponsored or endorsed by Blizzard and are related or connected to Warcraft 3."
Valve's response in original lawyer language:
"Valve is without knowledge or information sufficient to form a belief as to the truth or falsity of the remaining averments of this paragraph and, therefore, denies same."
Basically, they're claiming ignorance. Hopefully, this dispute can be settled quickly so it doesn't delay the game we now know as Dota 2.