A G++ from today's trunk (20110826) segv's on a reduced test case (using delta) like so:

bergner@igoo:~> cat minimal.ii 
typedef long unsigned int size_t;
namespace std __attribute__ ((__visibility__ ("default")))
{
  using::size_t;
}

typedef unsigned char uint8_t;
namespace std
{
  namespace decimal
  {
    template < class _Fmt > struct _FmtTraits;
    class decimal32;
  }
}
namespace std
{
  namespace decimal
  {
    template <> class _FmtTraits < decimal32 >
    {
      public:
      static const std::size_t _NumBytes = 4UL;
    };
    template < class _Tr > class _DecBase
    {
      uint8_t _Bytes[_Tr::_NumBytes];
    };
    class decimal32:public _DecBase < _FmtTraits < decimal32 > >
    {
      decimal32 () { }
    };
  }
}
bergner@igoo:~> /home/bergner/gcc/install/gcc-mainline-debug/bin/g++ -Wall -S minimal.ii 
g++: internal compiler error: Segmentation fault (program cc1plus)
Please submit a full bug report,
with preprocessed source if appropriate.
See <http://gcc.gnu.org/bugs.html> for instructions.

GDB show this backtrace:

#0  0x0000000010412cc8 in decl_is_template_id (decl=0xfffb6edcc18, template_info=0xfffb6edcc18)
    at /home/bergner/gcc/gcc-mainline-base/gcc/cp/mangle.c:267
#1  0x000000001041b44c in write_nested_name (decl=0xfffb6e24ac0) at /home/bergner/gcc/gcc-mainline-base/gcc/cp/mangle.c:928
#2  0x000000001041ab48 in write_name (decl=0xfffb6e24ac0, ignore_local_scope=0) at /home/bergner/gcc/gcc-mainline-base/gcc/cp/mangle.c:855
#3  0x00000000104284e4 in write_class_enum_type (type=0xfffb6edd4a0) at /home/bergner/gcc/gcc-mainline-base/gcc/cp/mangle.c:2398
#4  0x0000000010424840 in write_type (type=0xfffb6edd4a0) at /home/bergner/gcc/gcc-mainline-base/gcc/cp/mangle.c:1884
#5  0x000000001042d1ec in write_template_arg (node=0xfffb6edcc18) at /home/bergner/gcc/gcc-mainline-base/gcc/cp/mangle.c:2843
#6  0x0000000010428868 in write_template_args (args=0xfffb6f13a70) at /home/bergner/gcc/gcc-mainline-base/gcc/cp/mangle.c:2427
#7  0x000000001041b50c in write_nested_name (decl=0xfffb6e245b8) at /home/bergner/gcc/gcc-mainline-base/gcc/cp/mangle.c:932
#8  0x000000001041ab48 in write_name (decl=0xfffb6e245b8, ignore_local_scope=0) at /home/bergner/gcc/gcc-mainline-base/gcc/cp/mangle.c:855
#9  0x00000000104284e4 in write_class_enum_type (type=0xfffb6edccc0) at /home/bergner/gcc/gcc-mainline-base/gcc/cp/mangle.c:2398
#10 0x0000000010424840 in write_type (type=0xfffb6edccc0) at /home/bergner/gcc/gcc-mainline-base/gcc/cp/mangle.c:1884
#11 0x000000001042d1ec in write_template_arg (node=0xfffb6edccc0) at /home/bergner/gcc/gcc-mainline-base/gcc/cp/mangle.c:2843
#12 0x0000000010428868 in write_template_args (args=0xfffb6f14088) at /home/bergner/gcc/gcc-mainline-base/gcc/cp/mangle.c:2427
#13 0x000000001041b50c in write_nested_name (decl=0xfffb6e24ac0) at /home/bergner/gcc/gcc-mainline-base/gcc/cp/mangle.c:932
#14 0x000000001041ab48 in write_name (decl=0xfffb6e24ac0, ignore_local_scope=0) at /home/bergner/gcc/gcc-mainline-base/gcc/cp/mangle.c:855
#15 0x00000000104284e4 in write_class_enum_type (type=0xfffb6edd4a0) at /home/bergner/gcc/gcc-mainline-base/gcc/cp/mangle.c:2398
#16 0x0000000010424840 in write_type (type=0xfffb6edd4a0) at /home/bergner/gcc/gcc-mainline-base/gcc/cp/mangle.c:1884
#17 0x000000001042d1ec in write_template_arg (node=0xfffb6edcc18) at /home/bergner/gcc/gcc-mainline-base/gcc/cp/mangle.c:2843
#18 0x0000000010428868 in write_template_args (args=0xfffb6f13a70) at /home/bergner/gcc/gcc-mainline-base/gcc/cp/mangle.c:2427
...

The backtrace also show we have 10's of thousands of frames stacked repeated over and over, so it looks like we've gone into some type of infinite recursion.  A quick look at GCC 4.6 show's it fails in the same manner.  I haven't checked anything earlier.