Well, for starters, the Sleight of Hand skill says you can only "Lift a small object from a person" with a DC 20 check. However, I believe that assumes the object is not "attached" via necklaces or other things somehow. I'd certainly give some serious consideration if this even falls under the "can be picked off a person" category.
Anyhow, how I'd run it myself, is I'd allow the DC 20 check to lift it, but the cleric in question would get a +20 to the spot check to notice. (If the chain is broken then this is just a grab and run. The +20 to the spot assumes they actually lift it over the individuals head to not damage anything.)
Basically, the rogue can easily get it, but getting it without the cleric noticing is pretty damn slim.
Calrin Alshaw