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.

Test subjects:

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.