Sorry, I meant PSSM, not PSM. PSM/TSM and similar algorithms have a lot of temporal aliasing when camera moves. IMHO "normal" shadows look much better.
Yes, it's possible to implement SDSM on SM3.0. Just calculate depth range on GPU using pixel shaders and then read it on CPU. For read back use a few textures in round-robin fashion, so GPU won't be stalled by CPU. Results will be lagged by a frame or two, but it should look ok.