I don't think that will ever get fixed (unless you make a patch yourself). Phobos BigInt was created by Janice, and she seems to have completely disappeared. I'm working on Tango BigInt, which is completely independent, and will replace Phobos BigInt eventually. ( == as soon as we get a common namespace to put it into). I don't think anyone is interested in debugging Phobos BigInt.
Should still be bugzilla'd, so people know it's broken atm, and get notified when it's working. -- Simen