Mark reports: backtraces from x86 plts work; but from an x86-64 plt it still aports early.
See http://sourceware.org/ml/frysk/2008-q1/msg00173.html which provides basic support and enables the existing testcases. Looking through the abi and instruction manuals however I see there are other possibilities that aren't matches by the above heuristics. I'll try and find some examples and add testcases for those later.