Yes, the rumour was true. The Office of Film and Literature Classification has handed an RC to Bethesda's Fallout 3, making it illegal to sell the game in Australia. It's not known at this stage if the ban was for drug use, as the original rumour states, or gore as I was predicting.
I was ready to dismiss the rumour when I first heard it, but my suspicions were raised when Red Ant failed to confirm or deny it. It's not unusual for a publisher to do this, but it seemed particularly odd as Red Ant had no problem in the past commenting about Fallout 3 and the OFLC.
We'll be on Red Ant and Bethesda's backs to see what they plan to do. Most likely the game will be edited to remove the offending content and resubmitted, as AFA Interactive did with Dark Sector.
Update #1: According to Bethesda's Pete Hines, the developer has no comment regarding the ban. [Thanks briosafreak]
Update #2: A leaked OFLC report reveals exactly why the game was banned.