I'm about 98% certain a warrior resisted my cheap shot a couple weeks back. So I do think it's "working" to some extent. I will say I have no idea if the rate is actually 5%. I can only remember 1 resisted cheap shot in the last few months of playing rogue. After the game, I immediately jumped on the armory and confirmed he had Tense Unstable Diamond in his helm. On the contrary, I also have one equipped on my Druid, and I don't remember ever resisting a cheap shot (though maybe I wouldn't notice this in the heat of battle).
So I suppose the explanation is one or more of the following.
A) The resist rate is lower than 5%.
B) The number of people with this equipped is low.
C) I'm not observing every resist that's happens (possible, but when the target isn't cs, it really fucks up my plan on rogue so I feel I would notice this).
D) That one Cheap Shot I thought got resisted, didn't actually (maybe something else got resisted and the cs was immune?) and I'm completely out to lunch.
Which is a long way of saying, I think it's working, but maybe not at the stated 5% rate and I'm having a hell of a time confirming.