Customers will be able to download the beta release of Windows 7 until February 2009. After that date the download will no longer be available.
From the Windows 7 Team blog:
The shutdown of general availability for the Windows 7 Beta will occur in 3 phases over the course of the next few weeks: Starting January 27th, the Windows 7 page will be updated with a warning that time is running out on downloading the Windows 7 Beta and that we will be limiting downloads shortly. People will be encouraged to register and start the download of the Windows 7 Beta sooner rather than later. February 10th, new downloads of the Windows 7 Beta will no longer be available. People who have already started their Windows 7 Beta download and have not yet finished will still be able to finish their download and are encouraged to do so.
The shutdown of general availability for the Windows 7 Beta will occur in 3 phases over the course of the next few weeks:
You can find out more information here and you can download the Windows 7 beta (until February 10th 2009) from here.