I have 4 locks with a priest and love it. /Cheers to you. I got the Staff of the Purposeful Mendicant (http://www.wowhead.com/?item=38240) from Last Rites (http://www.wowhead.com/?quest=12019) and loved it and haven't looked back. A supposed upgrade (by levels) is the Chilly Slobberknocker (http://www.wowhead.com/?item=41821 ) from The Champion of Anguish (http://www.wowhead.com/?quest=12948 ). But I didn't really like it as much, just me though.

Lets see: (http://www.wowhead.com/?compare=41821;38240)
Mendicant Slobber
stam 52 66
int 39 62
spi 36 0

Others hit (to me, doesn't matter in PVP), haste (to me, doesn't matter with DOTs), SP (is higher, I will have to give it that one). It just seems like it isn't much of an upgrade for me. My priest wants spi for regen and has spi->SP talent, my locks have fel armor which is spi->SP. I hope I will find a easy quest staff at 80. I just don't think I will rush to 80 and worry about it. There are plenty of people that rush to 80, I want to enjoy the trip.
Also if you want to laugh at my lock, here: (http://www.wowarmory.com/character-s...n=Electhmwlock)