Just speculating, too (I never used multisampling, though I have Mesa 7.0 installed, where the X3100 driver supports GL 1.4), but I found this in the driver:
struct {
GLuint multisample_position_palette_index:3;
GLuint pad1:1;
GLuint num_multisamples:3;
GLuint pad0:1;
GLuint render_target_view_extent:9;
GLuint min_array_elt:11;
GLuint min_lod:4;
} ss4;
This seems to suggest they really support multisampling in hardware.
Philipp