This is simply a case of border control agents going 
  overboard

Again, and again, and again, and ... ? It's not an accident that these are the people being harassed. Again and again people involved in computer security, certain political campaigning and other 'dangerous' occupations are harassed. Far too often to be incidental.

Suggesting it's because of something they said is ludicrous and aggravating. That's the same argument as suggesting that if someone didn't want to get raped, she shouldn't have dressed skimpily. While you don't even know what she was wearing.

Is there a Godwin's Law equivalent for how long it takes someone to use a rape comparison in an online discussion?

Nothing is being compared to rape: an argument is being compared to a well known argument involving rape, to illustrate its fallacious nature.

Perhaps there is also a law for how long it takes until someone attempts to dismiss an argument by introducing a red herring.

It's an emotionally loaded comparison, even if you're referring to the argument and not the act.

I think untog brings up a good point, which is that we don't know what the interaction was. And we do know that what you say can cause you to be further questioned.

It's highly unlikely that so many smart people are being harassed 'because they said something unwise'. These people know very well to keep their mouth shut at the border, which is a sad thing in itself. How often is the person being harassed an accountant and how often is it a security researcher? Joe Random also complains on Reddit about being harassed and their stories are upvoted just as much. Still a few categories stick out. I can't explain that, unless they are being harassed, because of what they do or believe in when not travelling.

Of course, this harassment doesn't accomplish anything. If these people were truly a threat, the FBI would stake them out, the CIA would make them disappear or the NSA would hire them. Them being harassed at the border once more illustrates the weirdness of the TSA and border patrol. Their employees seem to suffer from the group delusion that harassing people will keep them from doing security research or being a danger to the United States. I don't understand their motivation. They give the US a bad name and they achieve nothing.

The actual security agencies know better than that. You may disagree with their practices and beliefs, but they at least give the impression of being mostly competent and efficient in achieving sensible goals. The current news around a 0-day cryptographic breakthrough being incorporated in Flame doesn't result in "I never expected that", but in "Go figure".

The introduction of rape as a metaphor serves an emotional purpose, not a logical one.

Working on encryption software : Being questioned at the border !:: Wearing a bikini : getting raped.

It's a lazy argument and he was right to point it out.

That's just contradicting me, by stating !::, where I'm obviously stating ::.

I think it's the best known example, and hence the best illustration, of 'blaming the victim'. I would have used that description, were it not for the fact that I couldn't remember the name of the fallacy at the time. I remembered it after a good night's sleep. If you wish to attribute my usage of an example to lazyness, so be it.

I wasn't going to let "You get temporarily detained at the border? Well, you must've said something that alerted them." slide and I don't see how it is logically different from "You got raped? Well, you must've worn something that aroused them."