I've read speculation that these people are what Paul refers to in 1st Corinthians 15:6 (ESV):
Then he appeared to more than five hundred brothers at one time, most of whom are still alive, though some have fallen asleep.
There are a few parallels:
- Many people involved
- Minimal details
- The euphemism sleep (κοιμάω) to mean death
- Occurred around the time of Christ's resurrection
- Provides evidence or support for the resurrection
But there just doesn't seem to be enough information to link these people together. It would be an ideal time for Paul to mention a mass resurrection since that was the point he was trying to argue in his letter. And if the resurrected people also saw Jesus resurrected, we might expect Matthew to have mentioned it. (Though this whole section is more of a side note to the main even: Jesus' crucifixion.)
I'm inclined to think that Paul did not know this story, since it would provide far more evidence of a general resurrection than what he did provide. Since Matthew was compiled later than any of Paul's letters, it's possible this story was not widely circulated at the time. It's also possible that Matthew and/or his source misunderstood what happened. There's no textual criticism evidence that I know of that the story was a later insertion.
This just seems to be one of those details in the Bible that we will never get the full story on.