I'm guessing the former. I know you have a bit of a hate on for Dumbledore, but I don't think it's plausible he'd ignore inside information about Voldemort. From HBP:

"When Voldemort discovered that the diary had been mutilated and robbed of all its powers, I am told that his anger was terrible to behold."

A subtle but significant turn of phrase. Somebody told him that. Whose job is it to find out what the Dark Lord is saying to his Death Eaters, remind me?

Actually, I think njelruch (below) has it right. Sirius was already in Azkaban when Snape joined the Order; he demonstrates in this very chapter that he wasn't aware Snape had been a Death Eater, and Snape in his turn is completely astonished, later on, when Dumbledore reveals that Sirius is working for him. I think the year at Grimmauld Place was the first and only time that they were ever in the Order together.