That could get tough if the unfamiliar ones were carefully-chosen to smell similar to the three main ones, for instance. I picked out those three because they're so easy to tell from one another (which is why the Pepsi Challenge things strikes me as silly) and because "blind" Pepsi/Coke taste-tests were the topic. I'd guess something like RC thrown in the mix could make me confuse it for Pepsi, and some of the closer Dr. Pepper clones might get me, especially on a smell-only trial. Coke'd be tricky since its got the mildest odor of those three and I'd have mainly picked it out by process of elimination. With the other brands thrown in I could well miss a few.

[EDIT] if it helps, it seems to me that having a "Pepsi Challenge" is like having an "Orange Challenge" where the other contender is a lime. It's not exactly gonna be hard to tell which one you're dealing with, even if the sample's the same size and you're blindfolded. Throw in a half-dozen other citrus fruits and yeah, I could see getting a couple of them wrong but just those two? No. Judging from the reaction here, though, I'm starting to wonder if I need to re-evaluate my sense of smell/taste. Maybe it's not only farther from normal than I thought, but in the opposite direction.