I had my iPhone die on me yesterday to where it would not respond to anything, would not turn on, would not do a thing. Then about 30 min later it turned back on and has worked fine since. Should I be concerned about this and are there probable causes that could cause this behavior that I should look into?
While not a definitive answer, I know someone with an iPhone 4 that did something very similar. After it happened several times, they contacted Apple support, and it was replaced under Apple Care. There's nothing that I'm aware of that you can or should do to help it, and if it starts happening more regularly, you should obviously contact Apple support.
I'm confident you already understand this but while it could be a mere coincidence, factors such as extreme heat, and water damage could possibly have the same effect.
Don't sweat it - if it's broken (or in the process of breaking) it will break thoroughly and completely in short order.
Do get a good backup (and understand not everything is backed up or needs to be backed up).
If it happens again - here are three relevant support articles to get you to the bottom of the issue: