Since the drop rate was decreased, then increased again to 1%-2% (according to armory), I've been solo farming PoS heroic on my Shadow Priest. I have dropped two hilts, sold for 10k each on my server (Karazhan EU).
I've not seen a Shadow Priest guide knocking about, so I hope this is useful to some of you...
Basically you're going to be killing the Deathwhisper Necrolytes at the starting area of the Pit of Saron for the hilt. There are seven of them in all, two on their own as you enter, and five wandering about with packs of skelly slaves.
The way you kill these as a Shadow Priest is simple but effective;
Shackle Undead on the Necrolyte and clear up any skelly slaves. Now the fight itself is simple, just use Shackle Undead to interrupt any Shadow Bolt casts by the Necrolyte.
You can open with Vampiric Touch, Mind Blast and Devouring Plague, by this time Shackle will have broken and he will be casting Shadow Bolt so you will need to interrupt with another Shackle.
In my experience I can only fit one spell in between Shackles, and every now and then a Shadow Bolt may sneak through, but one or two are easily absorbed and healed with Devouring Plague and Vampiric Embrace.
The only other spell the Necrolyte will cast is Conversion Beam, a longer channelled spell. I've found that nine times out of ten I can nuke through this channel, with Devouring Plague and Vampiric Embrace keeping me topped up, but if your health gets low, this can also be interrupted with Shackle.
Glyph of Scourge Imprisonment will redude the cast time of Shackle by 0.5 secs, making the interrupts a breeze. (thanks Creed)
So there you have it, make sure you have Shadow Protection and Vampiric Embrace buffed, and just keep up your dots while interrupting Shadow Bolts.
Good luck chaps.