Lee Chen, Senior Vice President at MLG, told GameAche.com that MLG "fully intend to finish their 2010 WoW season and that they are working closely with Blizzard to coordinate timing and availability of the Arenas in World of WarCraft: Cataclysm".
Reason for all this is because the Arena Queue master is not spawning at the 4.0.0.1 patch, and once Cataclysm hits Arenas will not be working at all.
MLG further explains that they were only able to host the Washington D.C. event with the help of Blizzard's special tournament realm which was patched to 3.3.5. Using the same realm for the Dallas event is impossible, as the hardware for that tournament realm is slated for Cataclysm server as soon as BlizzCon concludes.
When the national finals for MLG's World of WarCraft season will be played is as of yet unknown.
Links
GameAche.com - Source