What used to be the minimum required skill to fish in any given area is now the minimum required skill to not catch any junk. Once you meet or exceed this minimum for a zone, whether natively, or through the use of equipment, buffs and consumables, you will no longer catch ANY junk. If you are below this skill level, the chance of catching junk is proportional to just how far below it you are. The further below the threshold, the more junk you'll catch.
The exact formula for the chance of catching a fish when your skill is below the threshold is ((current skill/threshold skill)^2).
For example, the Dalaran Fountain has a no-junk skill of 525. As we can see in @GreatBigFish's answer, this junk threshold is above the (unbuffed) WOTLK maximum skill of 450. Fishing at max skill in Dalaran with no lure would yield a 'fish' roughly 73% of the time. The other 27% of the time, you will catch one of the 13 'junk' items found around the world, all of which vendor for mere copper. To never catch junk in Dalaran, you'd need to use a +75 lure or better at 450 skill.
There is one notable exception to this rule: You will never catch junk in a pool. Fishing from pools gives a 100% catch rate on whatever type of fish spawns in that pool. This has caused many to claim that actively leveling fishing skill is now pointless because it provides little practical benefit - fishing from pools is always more efficient anyway.
For more information, El's Anglin' provides a tremendous resource on the subject of fishing.
For protection warriors, Mastery is actually a great stat.
This thread on elitistjerks.com contains good theorycrafting data for protection warriors; particularly this section:
Survival Stat Priority:
Stamina > Mastery > Parry > Dodge
Stamina is still your best stat, but not to the extent of sacrificing everything else as was the case in WotLK. Conserving healer mana is a very real concern now. Mastery is your best stat for smoothing out incoming physical damage, and also increases rage generation through blocks. Parry provides the same avoidance level as Dodge now, but also procs the Hold the Line talent. Dodge will become more valuable as gear ratings climb higher to reduce diminishing returns losses from over-stacking the other stats.
For specific calculations, WarTotem over at TankSpot has made a lovely spreadsheet containing all the information you should need!
An important point is the "point of unhittableness" (ie when you have a combined chance of 102.4% to dodge, parry or block), at which point a boss can no longer "hit" you, only block dodge or parry. At this point mastery becomes an even better stat than it was before, since 60% damage reduction from a block is better than 30% damage reduction from a block!
You can also read through both threads for detailed discussions of the mechanics involved by people an awful lot smarter and more in-the-know than I am!
Best Answer
In general you should stack mastery until you hit the avoidance cap. Bosses get a bonus to hit, so total avoidance need to be 102.4%. Then the boss has a straight 5% miss chance so you want dodge+parry+block to get as close to 97.4% as possible. Once you reach that combined value, start switching from mastery to dodge and parry fairly equally (they suffer from diminishing returns so stacking one or the other is less valuable that spreading equally). The reason to try to cap avoidance with block initially is that even though it doesn't fully avoid the hit, it helps smooth damage spikes significantly, which is a big help to your healers.