Well, that all depends on what side you believe Snape to be on. If he is indeed loyal to Dumbledor, or perhaps more accurately Dumbledor really does completely trust him, then he would absolutely have to know about the Horcruxes simply because he would be a major part of finding them. He is a wonderful source of inside information. If you believe that Dumbledor really didn't trust him and he is working for Voldemort, then obviously Dumbledor is going to keep him in the dark.

There is one piece of evidence though that convinces me that Snape did know. When Dumbledor was injured, presumably destroying the ring, Snape was the one to prevent his demise. In fact, I think the text alludes that Snape is the only one familiar enough with dark magic to help. But then does the text specifically say that Dumbledor was injured destroying the ring, or was it that Dumbledor is just growing old and his reflexes have slowed, as Snape tells Bella in book six? I may just have to have a look-see.