This simple example fails when using polyglossia, complaining that \Btxtechreplong (billingsley74telescope is defined as a techreport in the bib file) is undefined (as if babelbib/bibgerm had not been included):
When using babel (which can cause problems in xelatex, especially when using hyperref) it works. Is there any way to get a correct (german) bibliography using polyglossia?
It's in the nature of the things that babelbib and polyglossia contradict. Since the first one explicitly is meant to work with babel, I'm not surprised about difficulties.