This is called cross-promotion, folks, and as soon as this tactic is seen as potentially profitable, you can expect several other movies to try the stunt. There has already been talk of a Superman vs Batman so the possibilities are endless for combinations of superheros to "visit" other superheros, which will help promote both movies at the same time. As Jason points out, its been done in comic books for a long time.
Rumors of the cross cameo have been rampant for quite some time now. It now seems likely that the ultimate goal is, in fact, to tie the “Iron Man” and “The Incredible Hulk” movies together and, more than likely, make an Avengers movie that would feature both characters (and likely Nick Fury who is played by Samuel L. Jackson in “The Incredible Hulk”). Jon Favreau, director of “Iron Man” has reportedly already called “dibs” on the Avengers movie.
Of course, this idea only works if you are using the same actors to play the characters in the other films. The recent dust up over the much-hyped "Justice League of America" using a different actor to play Superman, other than the recent incarnation played by Brandon Routh, could be seen as an example of breaking the suspension of disbelief.