Titanic is $600 million domestic, right? That..really doesn't seem out of reach. Avengers is almost two-thirds of the way there already. You're right that we're entering blockbuster territory, but Battleship looks weak, the Dictator is counter-programming just like Dark Shadows, and even Men in Black 3 doesn't seem like the sure-fire juggernaut it might have been. It seems entirely possible for Avengers to remain #1 for most of the month, and even when it sinks below that, it's going to keep making money. And it sounds like Disney is throwing its weight around to procure more screens from the various theaters.
It'll cross $400 million domestic by this Thursday, it'll probably be up to $500 million by the same time the week after, when the competition really starts to get serious...if it can hang on even a little I think $600 mill is within reach.
ETA: Hmm, OK, Titanic is $658 million. I agree that'll be tough, but again, doesn't seem completely out of reach.
Man, the discrepancy between Dark Knight domestic and foreign is weirdly huge. It just didn't catch on overseas. Avengers has already passed it worldwide but is still a long way from beating it domestic.