The gl_ClipPlane/p3d_ClipPlane inputs are in eye space (or “apiview” space, as it’s called by Panda’s Cg inputs system). This is different from the Cg equivalents, which are in world space.
Here’s a small sample program demonstrating how to use the gl_ClipPlane or p3d_ClipPlane input in GLSL. Sorry that the code is so ugly; I had quickly hacked it up to test the p3d_ClipPlane behaviour when I implemented it. It should get the point across, though.