Dude, when Loki trashed NYC, EVERYONE knew. And you also had the Cap, who was famous enough for a Smithsonian exhibit. Hulk remained a secret because the big guy's dangerous, and people are dangerous to Banner. That one wasn't for everyone else's sanity - it was to prevent Banner from coming to harm and to ensure another rampage doesn't happen.

See, if they aren't doing millions of dollars worth of damage in a large public area, it's possible to enact a coverup. [url=http://www.springhole.net/writing/more-believable-coverups.htm]Here's actually a writing article on how to write a coverup that goes into the logistics of it all[/url]. But frankly, this version would be the equivalent of covering up Godzilla.