Beast Mastery

I am going to start updating this information for 4.0.1 and Cataclysm.

Please visit the Beast Master Talents Preview page to learn more about the Beast Master tree.

Here is a  beast master leveling build to help you along your journey to becoming a great BM hunter.

Beast Mastery Spec:

Cataclysm 31/7/3: This is currently the most popular BM spec in beta. This spec focuses around getting the most damage out of your pet as possible. Your pet in BM does over one third of your total damage or more so this maximizes your pet.

4.0.1 31/5/0: This will probably be the popular 4.0.1 patch spec. I simply took out Pathing and Sic 'Em. It still has efficiency to lower focus costs as well as Go For the Throat to capitalize on pet damage.

Arcane Shot
Kill Shot
Kill Command, Rapid Fire
Since Arcane Shot is BM's most used shot it needs to be there. Kill shot I believe will be the one glyph ever spec has. It allows for 2 instant Kill Shots on a 6 second cooldown. The last glyph will kind of be up to the numbers. Reducing the focus cost of Kill Command is good and will probably outweigh the haste bonus during Rapid Fire. I would probably choose Kill Command.

Bestial Wrath
Trap Launcher
Misdirection I believe will be a glyph every spec uses. It allows you to instantly send some of your threat to your pet and immediately send more towards the tank allowing you to be able to focus on dps rather than threat. Bestial Wrath is just an obvious choice. It's a powerful glyph for BM hunters. The last one was hard to decide. I picked trap launcher just because nothing else really fit. Less focus to throw out traps might be useful.

Aspect of the Pack
Feign Death
Mend Pet
There isn't much to choose from, these are the exact same in all three specs. The others left don't really provide anything special.

Gemming (4.0.1):
Beast Master hunters gem for agility and ap/haste. There is no more armor penetration all the armor pen gems will turn into crit. 

Delicate Cardinal Ruby
Bright Cardinal Ruby

Rigid King's Amber
Quick King's Amber

Rigid Majestic Zircon

Glinting Dreadstone

Lightning Eye of Zul

Deadly Ametrine
Deft Ametrine

Nightmare Tear

Relentless Earthsiege Diamond

Socket Bonuses:
You should put your prismatic in your best socket bonus blue slot. Socket bonuses should only be activated if they contain a +6 agi or higher or +12 ap or higher bonus. Otherwise all red sockets should be agi, and all yellow sockets should contain deadly ametrine.

Gemming (Cataclysm):

Delicate Inferno Ruby
Delicate Carnelian

Fracture Amberjewel
Quick Amberjewel
Fractured Alicite
Quick Alicite

Rigid Ocean Sapphire
Rigid Zephyrite

Lighning Dream Emerald
Sensei's Dream Emerald
Lightning Jasper
Sensei's Jasper

Adept Ember Topaz
Deft Ember Topaz
Deadly Ember Topaz
Adept Hessonite
Deft Hessonite
Deadly Hessonite

Glinting Demonseye
Glinting Nightstone

Haven't seen any

Chaotic Shadowspirit Diamond

Socket Bonuses:
As of right now there is only one gem socket per piece of gear so you can put your prismatic into your best blue socket (if you have more than one). You can put hit in the other (if needed). Otherwise agi into red, mastery or haste into yellow, or put an orange if you would like.

See Cataclysm Enchants