I am totally having this problem. Mine will be 3 in September. All of a sudden, he is being much more difficult at bedtime too. He wakes up at least once a night, but usually for only a few minutes and goes back to bed, but sometimes for much longer. And if he wakes up past 5 am, then he is probably up for good - ugg. He also won't nap anymore. I know he is not getting enough sleep which is causing the problem, but we put him to bed as early as we can (start routine at 6:30). If we put him down later, he still wakes up at the same time.
We also have a second little one, born in April. Luckily, he sleeps most of the night, but only because we are co-sleeping right now. We've got to work on getting that one in his crib since I am a reluctant co-sleeper (with the baby not the older guy). Currently, it's just easier and I get more sleep.
The older guy has had a lot of changes lately though with the new baby. He also moved into a big boy bed and a larger bedroom, and we went cold turkey on the pacifier all in the last 3 months.
I just ordered a bunch of sleep books from the library. Hopefully someone has the answer.