I'm of two minds about comic crossover "events" -- while on the one hand it can be cool to follow a story so big that it can't help but have a major and lasting impact on the entire universe shared by the titles involved, the two comic giants responsible for pioneering this concept have definitely overused and abused it ever since Marvel's Evolutionary War extravaganza in 1987. That and not being a high school kid anymore, my monthly discretionary spending can barely accomodate picking up the latest trade paperbacks of Brian Michael Bendis' Powers and Y: The Last Man (as well as the occasional Astonishing X-Men), let alone the dozens of extra titles that are swallowed up by these mega-narrative marketing gimmicks.
Right now both D.C. and Marvel Comics have their own big summer crossovers going on. D.C.'s is called Infinite Crisis, and promises to turn the entire line of characters and titles on their collective head before it's all over. In the meanwhile there's a lot of betrayal and back-stabbing going on between the heroes and an equal amount of cooperation among the villains, with D.C.'s holy trinity of Superman, Batman, and Wonder Woman naturally at the center of things. Batman is fairly pissed that the Justice League has been messing with his mind and not telling him about it, an egregious breach of trust which lead him to create a super-surveillance system so as to keep an eye on his so-called "friends" that unfortunately has fallen into the wrong hands. A couple of major characters have already died, and rumor has it that several more will share the same fate as the story unfolds.
Marvel, on the other hand, has chosen an alternate reality crossover it's calling The House of M, in which Wanda Maximoff (a.k.a. The Scarlet Witch) -- the reality-bending daughter of mutant supervillain Magneto -- ends up creating a parallel universe in which mutants have inherited the earth, with Magneto as their benevolent dictator. The crossover reimagines the various Marvel titles in this context, where normal humans represent the underclass and non-mutant superheroes are regarded as dangerous freaks: Iron Man is a gladiator who fights killer robots for the mutants' amusement, the Fantastic Four is a supervillain group under the tutelage of Doctor Doom seeking to overthrow and replace Magneto, and Spiderman is a beloved hero hiding the dark secret that he's not a "real" mutant but a kid who just got bitten by a radioactive spider.
And they say comic books are just for kids...