Reflecting on my experience, I think I've come to four conclusions. These aren't words from God. They're just my ideas of what's okay and what isn't in sermon stealing.
Conclusion One: Everyone borrows. I don't know one preacher, one author, who doesn't get ideas from others. They don't steal, but it's impossible to be completely original. It's like the saying about someone who wanted to be original or nothing, and ended up being both. Some are more brazen than others, but even the most rigorous among us occasionally borrows from others.
Advertisement

I recently heard Andy Stanley preach on Romans 5-8. I loved the series, so I decided to preach my own. I think I used some of his ideas, but the sermons I preached were essentially my own. Everyone, at least occasionally, borrows ideas from others.
Conclusion Two: Sermon stealing isn't always wrong. There are times that it's okay to essentially steal a sermon. Church isn't school, and using someone else's material isn't plagiarism. Many preachers allow others to use their material within a church, as long as it's not republished. I think there are drawbacks to using somebody else's material, but it's not morally wrong, at least under some circumstances.
I can think of a couple of situations in which it makes sense to use somebody else's sermons. Sometimes bi-vocational pastors are left with little choice. Some pastors aren't strong preachers, and there is nobody else on staff to help out. One of my mentors told me, "Perhaps a lot of churches would be better off if struggling preachers used other people's material." I distribute my own sermons on the Internet, knowing that some other pastors probably preach them. I'm okay with that.
I've heard that Spurgeon sometimes preached other people's messages. I heard John Maxwell say that we should buy his sermons, because if we do, we not only get his sermons but Swindoll's sermons too. I don't know many pastors who haven't preached somebody else's message, at least once. It's not wrong, at least in itself.
Conclusion Three: We can't afford to hide it when we preach somebody else's stuff. I once attended a church in Florida with family. We heard the same sermon that my brother-in-law had heard in Holland. He left disillusioned with the church and with the pastor.
Somebody I know suspected that his pastor was preaching somebody else's material. He broke into his pastor's office and began to search the computer for evidence when the pastor walked in. That's a little extreme, but the point stands. We lose credibility with our people when they suspect us of passing off somebody else's material as our own. Some pastors have even been fired for this offense.