Since the Ether of the attacker is in a copy of a theDAO contract - can the ETH be stolen the same way?
This would have been possible if at least one honest actor would have voted for this split. Since the attacker is most likely the only shareholder and the curator of this DAO the same attack can not be done.
Quoted from the FAQ.
Update: Lefteris Karapetsas found out a way how to enter the "darkDAO" - this allows indeed to counter attack the "darkDAO" and all other potential thefts. This means that all ETH can be restored with soft forks only - read more here:
What this blog post wants to show is that there exists a way to stop the attacker from getting the money out even after the 27 days period has passed, but it relies on many moving parts and has several potential failure mechanisms.