weird, i have the same idle problem but it just doesnt die on me... yet....
half a year ago i noticed the idle problem just like u descibed, when i start the car engine has like 1400-1500 rpms and when it cools down it drops to 600, but agter i drive the car for a few mins the idle drops to 400, it doesnt matter if the tranny is in P or D but after revving the engine it goes away, but not always. i didnt really pay much attention since it is not causing any real trouble to me but if it will lead to the dying problem i should start to get worried right about now, i will check the numbers on my ECM on next weekend, too lazy and too cold to do it after work:)