Two reasons: 1. He's decided to take a ~8 month leave of absence as pastor of preaching and vision at Bethlehem Baptist Church. 2. He invited Rick Warren to speak at the 2010 Desiring God National Conference.
1. As an external observer, the decision to take an 8 month leave of absence and the way he's presented the leave demonstrate to me an excellent Christian example of humility and commitment to the one true God. He didn't sugarcoat it. He didn't call it a "sabbatical." He didn't try to baptize it and make it sound spiritual. He came out and told us the real issue: his pride is preventing him from loving his family and others the way God calls him to (it reminds me of David's attitude after his sin, Psalm 51 was written to the choirmaster, the guy who puts the stuff he's given out in the open for the assembled people of God (see v.1.)). I can only imagine the various lies satan was trying to feed him to prevent him from doing this. I tend to believe these lies myself: "if people see your failures, your ministry will fail," "don't do this, people will lose respect for you," "your books won't sell as well, you wouldn't dare want to risk that," etc. Especially as I've seen small leadership roles in ministry, my inclination is to hide my failures so people don't lose faith in me. But there's the problem: the goal of our ministry should be to direct people to faith in Christ, not faith in ourselves. Nobody grows in a great appreciation for Christ and His gospel by sitting under the teaching of a guy who presents himself as though he has no need of it. This decision of Piper's helps me to see him as less of a "Christian superstar" and as more of a sinner saved by grace. The former makes me worship Piper, the latter makes me worship Jesus.
It is for that very reason that I think this also gives a good example of commitment to the one true God. Piper has to know that this decision will hurt his superstar status in the eyes of many. If ministry success or his reputation were a real idol for him, there is NO WAY he could make this decision. If the approval of man and being put up on a pedestal by young Christians like myself was what he found life from, the idea of a leave of absence would be REPREHENSIBLE. The idol of superstardom is a tempting one, but I think Piper's actions here represent a casting down of that idol and a worship of the one true God.
2. I'll be the first to admit that this particular decision of Piper's is a bit more complicated. I'm personally torn on whether it was wise or not for Piper to invite Rick Warren to DG. On the one hand, I don't think Rick Warren preaches a different gospel. Therefore I think unity with him is worth pursuing and this is certainly a great way to do it. I also like the idea of giving him a chance to explain himself that will hopefully begin a conversation between Warren and the reformed community (an audience that normally wouldn't give him the time of day, myself included). On the other hand, I think in my limited exposure (and I do emphasize limited) to Rick Warren it seems he does tend to water down the full Biblical revelation of God in order to appeal to a broader audience. The unfortunate thing about this decision is I fear that no matter what Piper does to clear things up, history will view it as an endorsement of all of Warren's ministry (the good and the bad) from a majorly influential reformed theologian. On the third hand, John Piper is way older and wiser than I am, so I'm very slow to take a firm stance against him on a grayer issue like this. On the fourth hand, God is sovereign and this is a 3-day conference. It's really not the end of the world even if Piper's decision is less than wise. God will accomplish His purpose through it. The incredible backlash I've seen on the internet particularly from those in reformed circles really makes one question whether any of them really believe in the sovereignty of God outside of the ivory tower.
That said, I respect Piper for doing it. The reason I respect him is because he knew when he did it that he would receive criticism not only from the normal critics, but this time from his normally receptive reformed audience. If we're honest, much of what's being called the "new calvinism" is a fad. It provides a temptation to be more faithful to the fad than to what we feel God calling us to do. In this situation Piper had a choice between loyalty to the fad and loyalty to what he felt God was calling him to do: he chose the latter. Similar to his leave of absence, he had to know this would lower people's opinions of him in alot of ways, but he did it anyway. He doesn't view himself as part of a fad, neither should I.