Shortly after Microsoft released the Windows 7 Beta to the public earlier today, its servers were brought to their knees by overwhelming demand. If you tried getting the beta too late, you probably saw a generic "Server too busy" message. The surge in demand apparently had no immediate fix from Microsoft, and as a result they have decided to postpone the beta's release for now. Visiting the site now delivers a "Windows 7 Beta coming soon" filler page.
Microsoft is working on getting more infrastructure in place to handle the demand. It's not clear when they plan to put the beta back up, but given that they want several million testers (2.5 million at least), it likely won't be long. However, the folks over at HardOCP have posted a couple direct links for Windows 7 beta 32-bit and Windows 7 beta 64-bit which appear to be working just fine for now.