According to the GLSL "The variable gl_PointSize is available only in the vertex language and is intended for a vertex shader to write the size of the point to be rasterized."
So my understanding is that calling glPointSize shouldn't have any effect because me vertex shader will override the point size. What actually happen is the opposite: my point always end up being the one specified with glPointSize, and I'm unable to control the point size with my shader.
I first though of a driver problem, but I have the same problem on ATI and NV cards.
Any clue on how to control a point size from the vertex shader?
You need to enable GL_VERTEX_PROGRAM_POINT_SIZE_ARB.