Like four:love said it, I also think it is not surprising because I wrote and feared something like that already.
IMO the main reason for his decision is that the team was so succesful and happy without Kiwi last year and that Kiwi wasted a very good chance in February 2006 against France to make an impact on the team. He came with the biggest Grand Slam result of his career to Davis Cup, and could not prove it. And he also had some little issues with the team (not spending enough time with them, not appearing for a collective lunch or breakfast) which brought him bad news again. I'm sure some of you remember it. This was a huge blow, I think.
Kiwi really needs to continue like that. If he rises up in the rankings even more, it will be almost impossible for Kühnen to give reasons like this one.
But there was World Team Cup 2006 when Patrik Kühnen and Alex Waske praised Kiwi so much for leading the team so well and for his win against Gaudio. That was after the Davis Cup in Halle.
Anyway... if Kühnen is still offended by something that happened 2 years ago he should play in kindergarten but not work as Davis Cup Coach. Look at Barbara Ritter in Fed Cup. She nominated Analena Grönefeld to motivate her and show her that it's time to comeback. What did Kühnen say last year when Kiwi was injured? "For sure there is no place in the Davis Cup Team for Kiwi this year" not mentioning his comments at World Team Cup 2007 when he was hoping that Kiwi would be still injured and give his place in the team to Kohlschreiber.