Yesterday the Windows 7 beta was in a high demand. The demand was so high that Microsoft was not able to handle it. Right now they are adding new servers to ensure that all people can easily download the beta.
A direct download link to the beta has been removed in the meantime:
Thanks for your interest in the Windows 7 Beta. The volume has been phenomenal — we’re in the process of adding more servers to handle the demand. We’re sorry for the delay and we’ll re-post the Beta as soon as we can ensure a quality download experience.
The link above will probably work again later this day. If it does not, you can go to this site, where Microsoft will eventually post the link to the beta again.