This will sound like a weird one, but -- I have a 645, non-music, silver, and just noticed that all of my buttons are silver except the start/stop one. I really thought earlier that they were all silver, and was thinking a button cover fell off, but before I filed a defect report I thought I better double check, and now I'm even more confused. In some pics of silver 645s it looks like all the buttons are silver, in others just the start/stop is black, and in others still they all look black.
So - have a look at your silver 645 and tell me - which, if any, of your buttons aren't silver?