Blizzard is legally opposing Valve’s trademark for the Dota name which ‘for more than seven years has been used exclusively by Blizzard and its fan community, under license from Blizzard.’ according to Blizzard. Dota (or DOTA) stands for ‘Defence of the Ancients’, a fan made map for Warcraft 3. Valve hired two of the creators of the map (Eul and IceFrog) and trademarked the name Dota to use for their upcoming MOBA (Multiplayer Online Battle Arena) game, Dota2.
Update: Corrected myself. Blizzard isn’t suing just taking legal action in opposition to Valves trademark of Dota. The main point is still the same though, Blizzard are trying to stop Valve from using the Dota name.
Also posting what is apparently the Trial Schedule: