So what should you reforge?
As of cataclysm it is impossible to reach block cap, even with current raiding gear. While this will probably change as we progress further into the cataclysm tiers, for now the best thing you can do for survivability is reforge to mastery. You can safely reforge dodge, parry, or threat stats to mastery down quite low and still be gaining more mitigation through mastery. It is somewhat important to note that prioritizing the block cap in your reforging efforts may limit your tps output. Reaching the block cap will likely require the reforging of most of your gear pieces, leaving little room to reforge for hit/expertise if you don't have the right gear. In cataclysm, having a very low hit% is a given, especially in lower ilvl gear.
While some will choose to reforge for hit and expertise, My raiding experiences thus far have led me to advise the following: stack mastery in favor of dodge/parry to achieve as much avoidance percentage as you can. This means reforging away your highest stat (dodge/parry) to mastery on every piece of gear that isn't already reforged. Threat stats are almost trivial due to the high tps output we have mid to late fight, and the help of tricks/MD at the start of the fight makes it that much easier. If you maintain a proper rotation threat should only be an issue early in the fight, which is easily aided with tricks, misdirect, and AW