The issue (at least for Iron Man 3) has been addressed by Kevin Feige
"Iron Man 3 has been structured specifically to be the antidote to The Avengers," Feige said. "Circumstances in the story separate Tony from having access to anything.
"We wanted to take Tony back to, metaphorically speaking, the cave from Iron Man, the first half of Iron Man, when he's cut off from the world and needs just focus on his intellects to get himself out of his situation.
"So he's not calling Thor, he's not calling Captain America, he can't press a special button to have the helicarrier come rescue him, so I think that'll be a nice complement to the team-up of Avengers."
I wouldn't even think that would be an issue with Iron Man. Short of another alien invasion, Tony's ego would prevent him from asking for anybody else's help anyway.
To me the only issue future movies have to deal with is why Fury and SHIELD (who seem to be aware of everything that's going on) don't intervene and force Thor, Cap, etc to help out during one of those huge, city-wide battles that happen in nearly every superhero movie.
Thousands of lives are being endangered by some rampaging monster or supervillain, and Fury is just gonna let Iron Man or Cap handle it on their own? Really??