OK, I got it now. The full spec is at http://publib.boulder.ibm.com/infocenter/macxhelp/v6v81/index.jsp?topic=/com.ibm.vacpp6m.doc/language/ref/clrc03altivec_types.htm and the relevant passage is "Vector types can be cast to other vector types. The cast does not perform a conversion: it preserves the 128-bit pattern, but not necessarily the value. A cast between a vector type and a scalar type is not allowed." Andrew.
