As reported yesterday the Microsoft servers for delivering the Windows 7 beta ISO images have been overloaded pretty much as soon as the download page went up. It got so bad that Microsoft stopped the Windows 7 beta download and posted this message:
"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."

Update: Windows 7 Beta Download available again
There is no new release date and time given when Microsoft will commence the Windows 7 beta download.
You can check if the Windows 7 beta is available again on this page. Do not forget to check to see if you can even use the Windows 7 beta, before going crazy to try to download a copy.