As Khaz says we are now talking unhittable in Cata and simply put it's because at some point because of diminishing returns and dodge and parry caps block becomes more valuable than dodge and parry.
For pally tanks the theory has been well done by theck ... see
It boils down to solving an two equation equations to find the cross point.
Warriors get a different amount of block per point of mastery and our base model stats are slightly different so this work does not give us the exact answer.
I have not seen anywhere it has been done but given our blocks are less than the pally's but we have critical blocks and and we have a block CD that basically would make us block capped then we guessing is it will be close to around the same.
It's certainly not a requirement to tank at the moment and you can choose to ignore it it's just liek always alot of us are min/max'ing our survival.
** Remember Warcraft players fail in directions you never thought possible.