First, some of the "known good" snapshots of external projects that are used by the to-be-built utilities have gcc-4.3 related issues. Fortunately, Debian's bug and package database was very helpful, and I was able to extract the needed patches.
Second, gcc-4.3 added some new warnings, and the build system is set up to use -Werror by default. While some of the new warnings are helpful, others are not. Try, e.g., compiling this simple function with g++ -Wconversion -c test.cpp:
typedef unsigned char uint8_t;
void grow(uint8_t & u)
u *= 7;
test.cpp: In function 'void grow(uint8_t&)':
test.cpp:4: warning: conversion to 'unsigned char' from 'int' may alter its value
I.e., almost every in-place mathematical operation with shorts and chars results in warnings! With -Werror, this means errors in many files. Obviously, this isn't worth fixing, so I removed -Wconversion from the compiler flags in my checkout. Similar bugs are already recorded in GCC bugzilla.
Third, even after fixing fixable warnings and removing -Wconversion, I hit an internal compiler error. Since I found no similar reports (the error manifested itself on proprietary code, after all), I reduced the testcase and submitted it to GCC bugzilla.
After that, I gave up. I posted my changes to the internal code review list (just so that they don't get lost), switched the compiler to gcc-4.2 (it is available in Debian side-by-side with 4.3), and compiled what I needed without further problems.
BTW, Debian still uses gcc-4.1.2 for compiling their kernels. It would be interesting to know why.