The wizarding world is fairly small and insular, and I believe that in OotP, when Harry is looking at the Black Family tapestry and is surprised to find Draco Malfoy on it, Sirius says something about all the pureblood families being interrelated to some extent. If Severus was trying to pass himself off as a pureblood, shouldn't someone have said, "Hey, I've never heard of any purebloods named Snape"? Or, contrary to what was implied in that OotP scene, is the pureblood population large enough that an unfamiliar name would go unremarked? Perhaps everyone recognizes the oldest and wealthiest families like the Blacks, but there are enough obscure pureblood families that Snape's housemates wouldn't question his heritage. Or maybe they just assume that only a pureblood would be Sorted into Slytherin.

Going off on a slight tangent, since there seems to be a limited amount of pure blood to go around, I'm wondering if the unbalanced natures of some of the Death Eaters--Bellatrix being the obvious example--are due to upbringing or inbreeding? Is she just like that because she was indoctrinated with fanatic pureblood doctrine first at home and then in the DEs, or did her Black ancestors intermarry one too many times? Or maybe it's just her--Narcissa and Andromeda seem pretty normal, after all. I also like to chalk up Crabbe's and Goyle's stupidity to inbreeding. It just seems logical that if the same families keep intermarrying, sooner or later it's going to result in problems. In my Snupin fanfic series, which was started pre-HBP, when everyone still assumed that Snape was a pureblood, I had Snape's father choose a bride from a foreign pureblood family, precisely because he was afraid that the British familes were becoming too inbred. (Of course HBP came along and ruined that theory, but I don't mind writing AU.)

Getting back on track, if Snape's fellow Slytherins did know that he was a half-blood, how did he come to be accepted into the Death Eater crowd? Maybe Lucius saw Snape's talent in Potions and the Dark Arts, and decided that he would make a useful follower even if his blood wasn't as pure as it should be, and the others accepted him because Lucius did? I just have a hard time believing that the Death Eaters would knowingly welcome a half-blood into their ranks--I recall that Bellatrix refuses to believe Harry when he taunts her with the fact that Voldemort is really a half-blood. Even if for some reason, Snape was accepted as a half-blood Death Eater, due to either Malfoy's or Voldemort's approval, I would expect that the other Death Eaters would taunt him about his impure blood, for example in the Spinner's End chapter in HBP, when Bellatrix is questioning Snape's loyalty. She's distrustful and jealous of him, so I would expect her to make a remark like, "How can the Dark Lord trust a half-blood like you?" or something along those lines.

I don't think there's any right or wrong answer here unless, God forbid, Rowling chooses to give another interview to "clarify" loose ends in the series. (Sorry, that was a bit petty, but I couldn't resist. ^_^) But I am curious to hear other people's opinions on the subject. Personally, I'm leaning towards "everyone assumed Snape was a pureblood" even if I don't find it very logical.