Can't wait for this book.
Between this and Infinite Crisis, this summer is looking good.
Sure, Villans United #1 was kind of bleh, but Day of Vengeance #1 was pretty damn good. Omac looks to be good as well.
In a nutshell, Scarlett Witch goes crazy, alters reality, kills off a good chunk of the Avengers(thank god too, New Avengers rocks), and trashes the Avenger's Mansion...
Dr. Strange eventually shows up, tells everybody wtf is going on, and stops her.
House of M is basically the continuation of Avengers Disasembeled, as half the Marvel universe(Avengers, X-Men, ect..) tries to decide what to do with Wanda.
Actually, the main plot of House of M is apparently that Magneto uses the Scarlet Witch's powers to alter reality so that he rules the world and mutants dominate over humans. Supposedly, this will have major, lasting effects on the "real" Marvel Universe, unlike Age of Apocalypse.