Browsing from a machine in a trusted Windows 200x domain to a Windows 200x member of a trusting Samba domain, I get the following error:

The system detected a possible attempt to compromise security. Please
ensure that you can contact the server that authenticated you.

The event logs on the box I'm trying to connect to have entries regarding group policy not being applied because it is a member of a down-level domain.

If there is a computer account in the Windows 200x domain for the machine in question, and it is disabled, this problem can occur. If there is no computer account (removed or never existed), or if that account is still intact (i.e., you just joined it to another domain), everything seems to be fine. By default, when you unjoin a domain (the Windows 200x domain), the computer tries to automatically disable the computer account in the domain. If you are running as an account that has privileges to do this when you unjoin the machine, it is done; otherwise it is not done.