Alpha 1 is the first in a series of milestone CD images that will be released throughout the Natty development cycle. The Alpha images are known to be reasonably free of showstopper CD build or installer bugs, while representing a very recent snapshot of Natty. You can download it here:
http://cdimage.ubuntu.com/releases/natty/alpha-1/ (Ubuntu Desktop, Server)
Additional ISOs and torrents are also available.
https://lists.ubuntu.com/archives/ubuntu-devel-announce/2010-December/000793.html
