This works:
GL11.glMatrixMode(GL11.GL_MODELVIEW);
GL11.glPushMatrix();
GL11.glLoadIdentity();
GL11.glMatrixMode(GL11.GL_PROJECTION);
GL11.glPushMatrix();
GL11.glLoadIdentity();
GL11.glOrtho(0, 1, 1, 0, 0.1, 100);
GL11.glColor3f(255, 255, 255);
GL11.glDisable(GL11.GL_TEXTURE_2D);
GL11.glBegin(GL11.GL_LINES);
GL11.glVertex3f(0.0f, 0.0f, -1f);
GL11.glVertex3f(1.0f, 1.0f, -1f);
GL11.glEnd();
GL11.glEnable(GL11.GL_TEXTURE_2D);
GL11.glMatrixMode(GL11.GL_PROJECTION);
GL11.glPopMatrix();
GL11.glMatrixMode(GL11.GL_MODELVIEW);
GL11.glPopMatrix();
i.e. switch to Ortho-mode, use normalized screen coordinates and disable texturing.