There is a difference between spell hit and spell penetration, and that's what's prompted the change. At the moment, despite the tooltip saying that it increases your spell resistance, what the Cloak actually does is reduce your chance to be hit by spells. As a spell miss counts as a full resist, it's more or less synonymous in this context. However, no amount of spell pen will break through the cloak.
Spell hit, however, will. You'll need an awful lot of it for it to have a measurable effect, but that was what was happening due to casters going into PvP in raiding gear with excess hit. The 90% chance to miss was reduced to 60% or so - still a lot, but by no means immune. Hence many whines about "mages sheeping through cloak". This is of course very rare, for two reasons - one, the large amount of hit needed, and two, the fact that anyone with that amount of hit in PvP is clearly gimped in some other fashion.
In Cataclysm, it's changed to a 100% resistance/immunity/miss chance. Whatever the mechanic, spells will have no effect on you while you're Cloaked. This is simply to reduce RNG's influence on the game, and should have been the case from Cloak's release in TBC.
As to why it's now "200%", who knows. There are still lots of random, arbitrary placeholder numbers in the beta. There's currently a mage talent that increases crit chance of a spell by 4000%. Chances are it's just a number mashed in at random by a dev who simply wanted to make it definite and doesn't care about overkill. Numbers at this stage are in a state of flux, and nothing to worry about.
Hope that helped,