Adding to my other plea regarding integrating BitTorrent in iTunes, I’d like to add that I think that BT might make downloads much more robust over unreliable and slow Internet links. I am currently behind a really flaky Internet connection and there is just no way downloading larger files at all. Half way thorugh, downloads get terminated and iTunes tries to start all over again. Continuing http downloads from where they left off might be a way to do it, but this is not very reliable in case the target file changes on the server (I agree this does not happen often but I just did that myself a few times for various reasons and it must be taken into account).
Best thing about BT is that it splits up the file in smaller chunks that are each secured by a checksum. So downloads can be done incrementally and (more or less) securely.