I can't get the layout to work worth a damn but here is the basic information I collected and combined:

Lord Valthalak:  Lots of ranged DPS is helpful for this fight.
      I.	Phase 1, 100% to 41% health
         A.	Hunters, priests, locks – drain his mana.  Keep a mana sting on Val at all times, have a lock spam drain mana to keep it from building up.  (He heals &amp; in phase 2 uses mana for chain lightning effect)
         B.	Have all pets out and concentrated on Val (helps aggro adds).  One suggestion is to keep pets away from the boss and put them on aggressive to aggro the adds.
         C.	MT on Val, classes able to heal should do it and try to conserve mana:  MT #1 priority, locks &amp; mages #2.
         D.	Spectral Assasins will appear 2 @ a time during phase 1
            1.	Spectrals will freeze players &amp; channel.  After 10 seconds the player will die.  Everyone should turn on their mob healthbar (V by default) to better see these mobs.
2.	Everyone but healers &amp; mt should stop what they’re doing &amp; kill these as soon as they appear.
3.	Ideas:  arcane explosion (mage), multishot (hunter), instant hits (rogue, warr)
4.	When a player dies s/he becomes an additional Spectral
5.	One suggestion was using warstomp (tauren) &amp; shock (shaman) on the adds to stop them (??  Not sure if this would work after the draining spell starts…)
II.	Phase 2, 40% to 16% health
A.	Val changes weapons, Spectrals stop appearing
B.	All melee but mt run to the casters &amp; switch to ranged if possible.  This mitigates an exponential chain lightning effect (17k dmg)
C.	All out DPS, it helps if mt disarms.
III.	Phase 3, 15% to 0% health
A.	Val starts running around and attacking everyone – use shadow protection, he spams shadow bolts.
B.	Big time DPS
C.	Melee should run back in, prot warrior should use shield wall.  Maybe a tremor totem?