There seems to be a strong bias toward using Emacs, probably because Emacs itself uses a Lisp. There's no reason other (programmable) editors couldn't talk to Swank (the Lisp-side of Slime) though, and I'm not sure why people writing things like Eclipse plugins haven't taken that approach.
Yes, evidently it does. Seems strange that it would be tied to a specific version of SBCL then, or is that the user not knowing how to start Swank elsewhere and connect Cusp to it?