In ZODB, there is one consistency level. It should be possible to specify different isolation levels on a per-transaction basis to change the policy. We need to assess how many users still need Python 1.5.2 support. A ZEO server allows clients to connection using TCP sockets, but has no way to authentication the entity connecting. As a result, an attacker could connect to the server and modify the database.
Current practice is to use ZEO in a carefully controlled environment, i.e. behind a firewall.
Clients in different locations, e.g. east and west coast, could use the same server to serve content. Even inside a firewall, authentication could eliminate accidental misuse of a server. There are a variety of authentication approaches. SSL would allow both client and server to authentication each other. Within a single admin domain, Kerberos could be useful.