The Yanmar's that Deere has always used are as good as, or better than any diesel engine made. Period. I doubt they'll part ways with them any time soon. I'd take one over any Kubota any day. I have owned several, and still have one, and have never had a problem of any kind whatsoever with any of them. They are extremely dependable engines.
I believe that mowerbrad is right about them (Deere) testing the waters to see what response they'll get with these new Kohlers. they didn't even have them listed on their site last week. I was looking around at all their new fo '11 mowers and tractors, and not any mention of the Kohler cominng was made anywhere that I saw.