The following code produces different output in gcc4.0.0 and MSVC++7 .NET.
Additionally, I've tried 3.4.3 20050227 (Red Hat 3.4.3-22.fc3) and gcc 3.3.4
20040817 (Red Hat Linux 3.3.4-2), who all produce the same output as gcc4.0.0.

I'm unsure who is right here (don't know the exact C++ standards). If gcc4 is
right and MSVC++ is wrong this bug is invalid. 

CODE:
#include <stdio.h>

#define DEFSTR(name) \
	typedef struct name { \
		unsigned int var : 13; \
	};

#pragma pack(push, 1)
DEFSTR(pack1_struct)
#pragma pack(pop)

#pragma pack(push, 2)
DEFSTR(pack2_struct)
#pragma pack(pop)

#pragma pack(push, 4)
DEFSTR(pack4_struct)
#pragma pack(pop)

#pragma pack(push, 8)
DEFSTR(pack8_struct)
#pragma pack(pop)

#pragma pack(push, 16)
DEFSTR(pack16_struct)
#pragma pack(pop)

DEFSTR(unpacked_struct)

int main( int argc, char** argv ) {

	printf("sizeof(pack1_struct) = %u\n", sizeof(pack1_struct) );
	printf("sizeof(pack2_struct) = %u\n", sizeof(pack2_struct) );
	printf("sizeof(pack4_struct) = %u\n", sizeof(pack4_struct) );
	printf("sizeof(pack8_struct) = %u\n", sizeof(pack8_struct) );
	printf("sizeof(pack16_struct) = %u\n", sizeof(pack16_struct) );
	printf("sizeof(unpacked_struct) = %u\n", sizeof(unpacked_struct) );
	return 0;
}

PRODUCES ON GCC:
sizeof(pack1_struct) = 2
sizeof(pack2_struct) = 2
sizeof(pack4_struct) = 4
sizeof(pack8_struct) = 4
sizeof(pack16_struct) = 4
sizeof(unpacked_struct) = 4

PRODUCES ON MSVC++7:
sizeof(pack1_struct) = 4
sizeof(pack2_struct) = 4
sizeof(pack4_struct) = 4
sizeof(pack8_struct) = 4
sizeof(pack16_struct) = 4
sizeof(unpacked_struct) = 4