I'm having exactly the same issue!
My Sansa Clip+ goes to the 'Plug USB cord' screen. When I plug it into my Windows XP laptop, the Clip+'s screen changes to say 'Bootable USB mode', but it isn't recognized by RockboxUtility. After awhile the player changes to say *Panic* Mount: 0.
No matter what I do the RockboxUtility will not recognize that the player is plugged in, and I cannot see the player in My Computer.