How can one download the initial Blockchain data files securely? I see several sources for binary downloads of the Bitcoin blockchain.  Can any of these be used securely?
So far as I know, there's no conceivable attack model if you're going to use the block chain to seed the regular Bitcoin client. It won't accept an invalid block. And it understands that the "longest" block chain wins. So a compromised file that was longer than the official block chain cannot exist, and a compromised file that was shorter would just be rejected as soon as the client connected to the network.

On startup, the client checks to make sure the chain is basically valid and contains the most recent checkpoint.

If there is any attack that works in this way, it's a defect in the client. Because someone could send you the same "broken" blockchain over the network too.