As the title says i managed to solo The Kael´thas Sunstrider Encounter in Tempest Keep on my 85 DK on Beta.
This shows that the mount will be farmable in Cataclysm (but still quite hard with a lot of trouble)
I wiped ~ 5 hours on this boss
It isnt necessary to kite the encounter out of the room but without it you will need a lot of crit and resist luck to kill the adds of p3 and not getting CCd the whole time.
Enough said, see yourself and
Enjoy watching: http://www.youtube.com/watch?v=atATvXfxwPU
-Raegwyn
PS: Note that this is Beta and many things can change before release