"It so happens that I know of the plan," he said in a low voice. "I am one of the few the Dark Lord has told."
Some readers have speculated that he might not have, that he was bluffing. If that's so, that means he didn't know exactly what he was signing up for when he agreed to make the Unbreakable Vow. Why would Snape have taken such a risk? Possibly because he didn't know Draco's task and figured he darn well needed to know what Voldemort was plotting, so, by telling Narcissa "It's all right, you can speak freely with me," he'd have the chance to find out something crucial.
I don't see anything in that chapter that directly contradicts that idea; Snape's hesitation before he says the last words of the vow could also be said to support it. Perhaps he hesitates because he has no idea what he's promising, but knows he can't back out at that point without having his bluff called. However, one can also explain the hesitation if Snape does know that Draco's been directed to kill Dumbledore, because Snape would realize that by doing so, he's signed his own death warrant, since he's thinking there's no way he'll carry it through.
I'm more inclined to think that Snape does actually know, even though we have no hard-and-fast evidence. What we do have, though, is what can be seen as a clean example of an author's contrivance to keep the knowledge from the reader, by shrouding the element in "we must not speak of it, the Dark Lord has commanded," and "I already know of the plan." Keeps the reader in the dark. And that's all the explanation you'd need, as to why he says that. I think that's the most compelling element to sway me in that direction. But I do think that the chapter, and all the rest of the text, can support either hypothesis.