According to an email to beta testers, and  a blog post and a post on the adCenter website, adCenter Analytics will shut down on Dec 31, 2009, and no new beta participants are being accepted as of today.  From the blog post:

Our objectives at the outset were to serve the needs of small and midsize self-service customers, as well as evolve the Microsoft strategy to address the Web analytics space.

The insights you’ve contributed through your feedback and your use of the tool have served an invaluable purpose in shaping Microsoft’s future in this space. You’ve helped us work towards making an informed decision about building a general Web analytics solution, and despite the end of life plan, the beta was very much a success.

It enabled us to confidently determine that we can be of most value to advertisers and publishers by offering a tailored solution that meets more specialized needs.

adCenter Analytics started as a standalone beta called Gatineau in October 2007, and then was integrated into the adCenter website.  According to a comment from blog poster Mel Carson from adCenter Analytics, the closing of this beta does not indicate that the adCenter Publisher program, also in beta, is in danger of closing:

Publisher is NOT closing and is very much part of Microsoft Advertising's ongoing strategy in this space.

At the end of the beta period any analytics information will no longer be available, and beta testers are urged to save their information.  The blog post lists a number of alternative analytics products, more information on the blog post.