Writing REST services with Jersey (in Java) is what I do every day at work. I use the integration Jersey has with Jackson for JSON serialization and deserialization. Using another library with Jersey would be swimming upstream. You're using a REST library written in Java, so why not a JSON library in Java too, especially since it just works out of the box?
When I want to write a Scala REST service, I use SBT, the Scalatra web framework and lift-json. It works great.
I don't use Eclipse, but the selection of JSON library is orthogonal to what IDE/editor/etc you use. I would certainly never want to choose a library based on its ability to play nice in Eclipse.
If you're interested in using Scala to write a REST service (very quickly), start with this tutorial: http://blog.everythings-beta.com/?p=430
Add some case classes and lift-json support for parsing them. Excellent examples here: https://github.com/lift/lift/tree/master/framework/lift-base/lift-json/ (scroll down) You can add lift-json to your SBT project with:
val lift_json = "net.liftweb" %% "lift-json" % "2.1"
I've found lift-json excellent for both parsing and generating JSON. But if I'm using Jersey, I use the built in support for Jackson, since it is excellent and there's no reason to use anything else.