I neglected to introduce the Reputation concept in the blog post. As joeykrug said, Reputation is needed to report on events, and your report is weighted by the number of Reputation tokens you have. So you can't sybil attack simply by splitting your tokens across multiple accounts, as this wouldn't amplify your reports in any way.
The other key point is that Reputation holders are not able to report on events as often as they want to. Instead, they only can report on events at fixed, pre-defined intervals. This precludes sybil attacks by simply report-spamming the network.
This is a very important point, and is something I (unfortunately) glossed over in the blog post:
In Augur, truth equals consensus. As such, the Augur oracle is intended to be used for events which are easily and objectively determinable after the event has occurred. (For example, the winner of an election.)
However, as you correctly point out, there are MANY cases where consensus might not reflect the truth, such as:
- Events where there is ongoing controversy about what happened (e.g., "Malaysia Airlines Flight 370 was brought down by terrorists")
- Events where the outcome is subjective (e.g., "was Carter a good President?")
- Events where the outcome is unreasonably difficult to determine (e.g., "what is President Obama's checking account balance?")
These events are NOT good candidates for Augur! In fact, all questions include a "this was a bad question" answer, in case a user is asked to report on an ill-defined or unanswerable event.
Of course, Augur's oracle can dutifully report the consensus in these cases -- and the consensus very often will be "this was a bad question" -- but it's up to you to use your judgment as to whether this consensus is an accurate reflection of the truth.