problem with WP 3.2.1

Even after successfully registering and logging in, UAM does not recognize that a user is logged in.

I have noticed this:
Until recently, my multi-site installation worked great. I had people register for the site, and when they registered they were automatically ‘assigned’ to the main site.

Now, they are no longer automatically ‘belong’ to the main site. And this is causing some problems. I’ve seen this exact same problem now on 2 separate sites, even after I get rid of all plugins.

Any ideas out there?