Notice they don't play the lyrics with the "Iron Man" song in the closing credits. That's because the Black Sabbath song was not about the comic book Iron Man. There are many theories about what the song means, but the most popular is that it is about a man who travels to the future and sees the apocalypse. They couldn't use the lyrics to describe the Iron Man in the movie: He was turned to steel - In the great magnetic field - Where he traveled time - For the future of mankind. Now the time is here - For Iron Man to spread fear - Vengeance from the grave - Kills the people he once saved.
Director Jon Favreau appears as Happy Hogan, one of Tony's personal assistants. [It may not be as notable as Stan Lee's cameo, but as Favreau is the film's director, I think it's significant enough to point out.].
The movie has some interesting hidden promises: Downey will appear in The Incredible Hulk (again as Iron Man), while at this movie's conclusion, the Avengers are mentioned. Thus, whereas the previous Marvel features (X-Men, Daredevil, Spiderman) are as yet essentially stand-alones, the Iron Man movie initiates the first character cross-overs in the Marvel Comics movie conversions.
When Pepper Potts is stealing Obediah's files off of Tony Starks computer in his office, pay close attention to some of the names of the files. On several files that show on the computer screen, a few are labeled "Lebowski". A playful homage to Jeff Bridges (who plays Obediah) and his famous slacker character from the movie of the same name.
In the scene where Col. James Rhodes briefs the media about the "training exercise" incident, if you look at the microphones at the podium, you can see three from left to right, 10 News, 19 KJFM and 6 KPGB. The first two are facing flat revealing the numbers 10 and 19, and the 3rd is at an angle so you can see the 6 twice, making, 10-19-66 or October 19, 1966, the director Jon Favreau's birthday.
The reason Starke's friend Rhodes looks at the suit of armor at the end of the movie and says something like "Next Time!" is because in the comic books he ends up actually having his own suit of armor, donning the name War Machine. He backs Tony Stark up in many situations and is one of his most trusted friends.
The cars in Stark's garage are from left to right: a grey/black 2010 Audi R8, a blue/white AC Shelby Cobra, an orange Saleen S7 and a grey Tesla Roadster. Next to the Tesla are two motorcycles custom made by Zero Engineering.
The call sign of the lead F-22 Raptor pilot is "Whiplash One". Whiplash is the name of the main villain in Iron Man 2.
During the first part of the end credits we see a simple animation that eventually resolves itself into the symbol of SHIELD. Part of this animation shows not the Iron Man, but the War Machine armor (note the shoulder-mounted external machine gun, for example) that Tony Stark constructed for James Rhodes in the comic book. This is the second hint in the movie at Rhodes becoming War Machine in the future (the first being Rhodes glancing at an unfinished armor and saying, "Next time").