I can't see this posing a serious threat to Bitcoin. If I understand this correctly the idea relies on mining a block and keeping it secret so that the selfish miners can start mining on the next coin. But they can't change the properties of the network. They'll have to respect Bitcoin's difficulty calculation, since if they try to change that, the main Bitcoin network will reject their chain. Plus, they'll need tons and tons of hashing power to stay ahead of the network.

Correct me if I'm wrong, but even if you've got future blocks precomputed for weeks in advance, all it takes is someone else finding the current block by chance to completely invalidate your private chain?