Windsong

iamleejn
[spell=104423] is the related haste buff.
[spell=104510] is the related mastery buff.
[spell=104509] is the related crit buff.

Currently on the Beta, it is possible to have all 3 buffs active at once.
There is no internal cooldown on the buff and which of the 3 you get each time is random, not based on your current stats.

The devs told us exactly how MoP weapon enchants work at the link below.

http://us.battle.net/wow/en/forum/topic/6397900436?page=8#154

I'm pasting it here in case they retire the beta forums after release.

• 
  
• River's Song: 4 RPPM on melee attacks that land, or are dodged, or parried.
• 
  
• Dancing Steel: 2.3 RPPM on melee attacks that land.
• 
  
• Colossus: 6 RPPM on melee attacks that land, or are dodged, or parried, with a 3-second cooldown.
• 
  
• Elemental Force: 10 RPPM on damage or spell damage/healing
• 
  
• Jade Spirit: 2 RPPM on spell damage or healing, 50-second cooldown.
• 
  
• Windsong: 2 RPPM on damage, or spell damage/healing.
• 
  

Edit: As of 10/15/12, Windsong and Elemental Force now proc on all damage and healing, including melee, ranged, direct and DoT.

They now trigger on the new "Real Procs Per Minute" (RPPM) system, which is weapon speed and instant attack independent and fully detailed in this blue post.

http://us.battle.net/wow/en/forum/topic/6893549789

Edit 2: As of patch 5.1 Dancing Steel, Jade Spirit, River's Song, and Colossus are on the RPPM system also.

Edit 3: As of patch 5.2, Dancing Steel now has a 15% greater chance to activate.

Source: http://us.battle.net/wow/en/blog/8896363/52_The_Thunder_King_Patch_Notes-3_5_2013

*Update for this enchant, Blue-repost from Daxxarri here and here on 10/15/2012.
--------
Quick (new) facts are:

• 
  
• Windsong is 2 Real PPM.
• 
  
• Regardless of how you’re attacking or healing, slow or fast, with DoTs or direct heals, whatever, you can expect to get the same proc frequency, on average.
• 
  
• Dual wielding and having both weapons enchanted with the same enchant will double the frequency of procs that you get.
• 
  
• This Real PPM is increased by your haste %. (The highest of your melee, ranged, or spell haste is chosen).
• 
  
• Simple as that. Whether you’re an Affliction Warlock dealing very frequent DoT ticks and [spell=103103] ticks, or a Holy Paladin casting purely [spell=635], or a Combat Rogue quickly attacking and using specials, or an Enhancement Shaman attacking with slow melee attacks and spells, or a Shadow Priest channeling [spell=48045] on fifty Onyxian Whelps, you’ll get 2*Haste Windsong procs per enchant per minute.
• 
  

Simply now everyone will just about benefit from this enchant, enjoy!
*edited to fix links*