Teheheman said:
Maybe it was a sign cause he couldn't really help her anyways(She was gonna die anyways) and he teleported back. Either that, or he can only stay there a certain amount of time. Remember, he couldn't stay long enough to tell Phil why he had to save the cheerleader. Speaking of that, they saved the Cheerleader, now what? What happens now? Have they saved the world?
I had the same notion about the waitress, Teheheman. Even if he tried, he couldn't save her (from Sylar or her health problem), which made it such a sad ending to a sad side-story. And you're right about the fact that he can only stay for a certain amount of time.
I think the reason he couldn't travel back or teleport afterwards is because he must've drained a lot of energy out of himself, traveling six months in the past and even using his abilities while there as well (in one case stopping time to make a hundred paper swans). Hiro's powers must be like a battery, meaning once it's recharged, he can start all over again.
And I also think we're going to get the answer about that "Save the Cheerleader, Save the World" thing next week, when all of them come together in New York to save it from exploding. My guess is that it means that once the heroes were able to save Claire, then they can unite to save the world.