how are you rendering?

like alan said, if you increase your dmap resolution (don't remember what the default is at but you could probably go to 1024 and not sacrifice render time too much) it should smooth out the blockiness...

you could also increase the dmap filter to a higher number. that won't so much get rid of blockiness but moreso blur it up a bit... try it anyway though.

(oh that's for if you're using depth map shadows on your point light...)