As for the bonus scene at the end of the credits, I won't reveal anything except to say that Marvel seems to be laying the groundwork to start having cross-over movies involving multiple heroes. That will be really nice if they do it well.
Nitpicks in the comments.
Update: There's a trivia item from the imdb.com write-up of Iron Man that just has to be shared...
Robert Downey Jr. further described his portrayal of Stark as "a challenge of making a wealthy, establishmentarian, weapons-manufacturing, hard-drinking, womanizing p#&%! into a character who is likable and a hero."