Mastery information is available here.
http://spreadsheets.google.com/pub?key=t1fF8IXJ-xJycjxizYU2o9w&output=html



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For example, a single point spent in your shaman's elemental tree will give the following effects:

    * Spell Damage + 0.157%
    * Spell Crit + 0.157%
    * Chance to proc Elemental Overload + 0.5%

Doesn't seem like much. When you've capped out your tree's mastery points at 51, though, the numbers look much better, rounding off to about the following:

    * Spell Damage + 8%
    * Spell Crit + 8%
    * Chance to proc Elemental Overload + 25%

But there's more! The mastery stat, which you'll find on gear in the new 78+ Cataclysm zones, will also augment your third "major" mastery bonus further. Pretty neat trick, making a stat that's useful for every class and does 30 different things.

With the knowledge that the expansion is still very much in the testing phase and these numbers likely aren't final.