During the weekend Microsoft had to temporarily take the beta version of Windows 7 offline, as its servers couldn't handle the massive demand. At the moment the Windows 7 Beta is available again, and Microsoft has decided to remove the initial 2.5 million limit on the public beta until January 24th:
Due to an enormous surge in demand, the download experience was not ideal so we listened and took the necessary steps to ensure a good experience. We have clearly heard that many of you want to check out the Windows 7 Beta and, as a result, we have decided remove the initial 2.5 million limit on the public beta for the next two weeks (thru January 24th). During that time you will have access to the beta even if the download number exceeds the 2.5 million unit limit.