Yeah, that's true, and I'm actually wondering a bit whether she might be asked to occupy the CJ chair until that vacancy is filled. (musing out loud here).
1. Roberts to replace O'Connor
2. Mystery Candidate to replace Rehnquist on the court.
3. A sitting justice (Scalia, for example) to become the new Chief Justice.
There would only be two hearings if the Mystery Candidate were nominated as the new Chief Justice. I find that unlikely, though.