OK, formally it's not undefined but unspecified. But for evaluation order, that basically doesn't make much difference. The draft I have (N1256, which is basically C99 + TC3) specifically says so in section 6.5:
"The grouping of operators and operands is indicated by the syntax.74) Except as specified later (for the function-call (), &&, ||, ?:, and comma operators), the order of evaluation of subexpressions and the order in which side effects take place are both unspecified."