g++ compiled as of yesterday afternoon:

Using built-in specs.
Target: i686-linux
Configured with: ../trunk/configure --enable-languages=c,c++ --build=i686-linux --host=i686-linux --target=i686-linux --prefix=/g/users/cgd/proj/gcc-trunk/bld/../inst
Thread model: posix
gcc version 4.4.0 20081123 (experimental) (GCC) 

rejects the AFAICT-valid code:

#include <bitset>

class C0 {
 public:
  C0() : b(0) { }  // FAILS.
 private:
  std::bitset<1> b;
};
  
class C1 {
 public:
  C1() : b(1) { }  // OK!
 private:
  std::bitset<1> b;
};

void func() {
  C0 val0;
  C1 val1;
}

(in particular, the 'C0' initialization) with the error:

test.cc: In constructor 'C0::C0()':
test.cc:5: error: call of overloaded 'bitset(int)' is ambiguous
/g/users/cgd/proj/gcc-trunk/inst/bin/../lib/gcc/i686-linux/4.4.0/../../../../include/c++/4.4.0/bitset:808: note: candidates are: std::bitset<_Nb>::bitset(const char*, char, char) [with unsigned int _Nb = 1u]
/g/users/cgd/proj/gcc-trunk/inst/bin/../lib/gcc/i686-linux/4.4.0/../../../../include/c++/4.4.0/bitset:744: note:                 std::bitset<_Nb>::bitset(long unsigned int) [with unsigned int _Nb = 1u]
/g/users/cgd/proj/gcc-trunk/inst/bin/../lib/gcc/i686-linux/4.4.0/../../../../include/c++/4.4.0/bitset:652: note:                 std::bitset<1u>::bitset(const std::bitset<1u>&)

I'm not completely sure what's going on here, but I'm thinking it's related to the fact that the 'char*' ctor actually has two default arguments, and 0 is convertible to a pointer.

This code is accepted by GCC 4.3.2 and the Comeau test drive.