no, all my music files are mp3, and they worked on the same iphone 3 before the move to the new laptop.

Any time I have run into that problem (on Droid, not iPhone) it was because the file was in an unsupported format. Could it maybe be a .wma rather than an mp3?

What does it mean when MM4 "send to" copies a mp3 file to my iPhone, the phone says 'synch', and the track appears in MM listed under the device, BUT, try to play it from there, turns it grey and skips to the next song on the phone? And even tho the iPhone said "library being rebuilt", the track is not there?

Is it related to the iTunes problem where I try to "manually manage music" as advised here, and iTunes says "sure just let me erase everything on your phone first, because it's synced with another library"? Which I of course decline. I don't know what or where that other library is, I've avoided iTunes like the plague it is, so it's probably been melted down for salvage long ago and I was supposed to transfer [whatever it is wherever it is] to each new pc :evil:

iPhone is 3G, ios is 4.2.1, iTunes is 10.1, MM is 4.0. (these are compatible, yes?)
The wife's 4G does not have this problem, nor does iTunes refuse to recognize it without erasing it first.