Let's just play this safe and assume that, until more samples have been collected and detailed DNA analysis has been done, the real answer to the question, "Is bacteria found in Antarctica's Lake Vostok actually new to science or just contamination from the drilling?" is "We don't really know." This is a great example of why making scientific pronouncements from the field, before you've had time to do the really in-depth analysis that goes into writing a peer-reviewed research paper, can be problematic. Right now, you've got different camps of researchers making totally contradictory claims. Who is right is, so far, anybody's guess.