Here's a plausible story and some technical evidence (see the HDD ID codes as ST = Seagate 5GB "ST1" drive, and HM = Hitachi Microdrive) that recent iPod mini's may already have the Seagate 5GB drive, and are simply limited via firmware to the advertised 4GB.
http://www.ipodlounge.com/forums/showthread.php?s=&threadid=65447&perpage=15&pagenumber=1
http://www.ipodlounge.com/forums/showthread.php?s=&threadid=65447&perpage=15&pagenumber=1