With MASM 9, using the MMX unpack instructions with a pointer argument explicitly marked with "qword ptr" now results in A2070: invalid instruction operands. This is an undocumented breaking change from MASM 8 (VS2005).
My understanding is that a lack of a pointer prefix means that the memory argument size is supposed to be implied from the register argument, which means that the meaning should be the same with and without qword ptr. This is why movd mm0, [eax] not assembling with MASM 8 was deemed By Design. This means that adding "qword ptr" here shouldn't make a difference, but it does.