Jantje:
I'm an IT guy not a electronics guy; so I don't dare to say you are wrong but the info you provide is seem conflicting to me with other info available.
When looking at he schematics http://arduino.cc/en/uploads/Main/Arduino_Uno_Rev3-schematic.pdf I can not conclude Vin and USB power line are connected.
USB power feeds the +5v line via mosfet T1, initially through its body diode. If there is no supply attached to Vin then the output of U5A goes low, turning T1 on and providing a low-resistance connection between USB power and +5v.
If you power the Arduino with +5v from a regulated PSU and also connect the USB port to as PC, then the output of the PSU does get connected to USB power, which is not such a good idea, although the fuse provides some protection. Powering the Arduino from a higher voltage applied to Vin or the barrel jack is safer, because under these conditions T1 will not be turned on.