Daniel has posted an interesting response to Roger Olson's attempted use of material from Vincent Cheung. On this topic of God being the "author of sin," the one positive point that Cheung has brought to the table is that he makes (or ought to make) folks like Olson stop and try to explain why it is wrong to call God the "author of sin."
For example, if by "author of sin," you mean that God permits sin to happen for some higher reason, then how would that be a wrong view? Of course, that's not the objection. The objection is typically raised against a view that God decrees sin to occur.
But is such a view equivalent to making God the author of sin? Again, it depends how you define that term. If you define it to mean that God has moral culpability for the sin, then no - Calvinists don't believe that, Calvinism doesn't teach that, and Calvinistic views don't imply that.
Or is something else meant? In any event, in these debates we need to force the opponents of Calvinism to explain their objections for the sake of clarity, rather than getting caught up with ambiguous or equivocally understood expressions.