At least, according to a chat Chris Pirillo had with MS Mindshare Manager Aaron Coldiron. Coldiron said:

We are pumping out bits as fast as we can. If we pushed out bits any faster there would be a measurable impact for the Internet. So, we are literally saying that if we increased our bandwidth any further there’s a possibility of taking down the Internet

When Chris asked why they couldn’t just seed it as a torrent, Coldiron responded:

There are legal and privacy issues which unfortunately make that not an option for Microsoft to officially sponsor a BitTorrent. I really wish we could do it, but we can’t. If someone [seeds or downloads a torrent] we can’t guarantee that they’ve got an unaltered copy, etc.

This is obviously bullshit, Legal and Privacy issues? Sounds to me like Microsoft just isn’t capable to produce an MD5 hash, like Chris Pirillo already noted.

Update: BitTorrent.com and vistatorrent now provide clean Vista Beta torrents

Source: TorrentFreak, for the latest info on copyright, file-sharing and anonymous VPN services.