Today? Not at all. What I've been thinking about though is if we have something like 1GB blocks. A scheme like this could allow peers to start sharing the block out across the network without having to first wait to download the entire block. I'm trying to think of how we could reduce the latency involved with getting huge blocks out across the entire network.

Oops, only replied to half your comment. I wasn't thinking of trying to verify that later chunks of the block were valid before you have the earlier parts, just that the chunk did in fact make up a piece of the block, and that it wasn't just random garbage. You'd still have to validate the block itself in order.