I just wanted to very briefly comment on Orin's post on the subject. Dixon v. United States involved the question of who is to bear the burden of proof as to a duress defense. The "long-established common-law rule" had been that the defendant must prove duress by a preponderance of the evidence, and the Court held that Congress did not intend to displace this rule. This is where the "offense-specific context" language comes up (citation omitted):

Congress can, if it chooses, enact a duress defense that places the burden on the Government to disprove duress beyond a reasonable doubt. In light of Congress' silence on the issue, however, it is up to the federal courts to effectuate the affirmative defense of duress as Congress "may have contemplated" it in an offense-specific context. In the context of the firearms offenses at issue -- as will usually be the case, given the long-established common-law rule -- we presume that Congress intended the petitioner to bear the burden of proving the defense of duress by a preponderance of the evidence.

It seems to me that this common-law tradition is the most important factor here, and the longstanding common-law acceptance of the defense-of-property defense should lead federal courts to assume that Congress didn't mean to preempt it, at least absence a statement from Congress to the contrary.

It's true that Congress likely didn't think much about the defense when enacting computer crime laws; but the point of the common-law criminal defenses is precisely that the legislature often doesn't think much about defenses, which often (as with duress, for instance) involve relatively rare circumstances. The defenses are out there to be used when the triggering circumstances arise, and Congress doesn't need to think much about them when enacting specific statutes.

So it seems to me that Dixon is quite consistent with my position: Congress legislates against the background of various common-law rules related to criminal law defenses, and the general presumption is that Congress doesn't mean to displace these background rules.