I was doing some testing on this today before raid, because I hadn't read any conclusive proofs yet. My apologies if this has been seen elsewhere, I searched but didn't find it.
The goal is the figure out whether the 2 pc bonus from the blue pvp gear stacks with the 2 pc bonus from the arena gear. Both bonuses are +35 resilience. I've heard varying theories, and heard from GMs that although the set bonus doesn't display in your character sheet, it does work.
Rogue, naked(just in case)
Warlock, 187 tooltip resilience(222 if the 2nd set bonus works)
First test: 0 Resilience
I had the rogue build 5 points and expose the warlock down to 0 armour. He then sat down, and was kicked. Result was a 229 crit.
If the set bonus works, he'll have 222 resilience. This would result in a crit of 203.581.
If the set bonus does not work, he'll have 187 resilience. This would result in a crit of 207.5885.
Second test: 187(222) resilience
Same test was done. Result was a 207 crit. Therefor, the set bonus does not work.
To figure out how much he should be crit for, I took the base damage of 229, and multiplied it by the expected damage reduction. 229 x (1-(222/20))
As a final precaution while I was writing this out, I added up all his resilience manually, and it matches up perfectly. 187 if you were to not count the second bonus, or 222 if you were.
Conclusion: The set bonus does not apply to your character sheet stats, nor does it actually provide the bonus.