Satoshi,
The “block lock” is an interesting idea, and I’ll have to think more about it. However, it’s important that you don’t lock all the way up the very latest block. Otherwise, the attacker could generate a fake block (or a few) right before you happen to lock it, and then his attack would be far easier than it would have been without the block lock.
Instead, you should only lock in everything except the last thousand or so blocks (the exact number should be a subject of future inquiry). Maybe this is actually what you did?
Also, I’m assuming that the block lock means that the blocks will also come prepackaged with the client. Is this so?