a) It's still there on a clean device, and I think I know how to pair a bluetooth device.
b) In such case the n900 manufacturing yield is not the best.

Well, maybe you know more than me what regression test strategies they use at nokia, but if they are testing their bluetooth stack on just a few devices, they are *****ic. The bluetooth standard may not be interpreted in the same way for all products like the ones from Nokia (or even the specific n900 development team at nokia). It was not a specific brand or device that failed in this regard.