Grant column level permissions

Hi, from postgesql features list mentioned at http://www.postgresql.org/about/press/features84.html, I came to know that it is possible to grant column level permissions.

PostgreSQL is "the most secure by default" and part of that is making security tools easy to use. 8.4 makes our existing connection and access control more flexible and simpler to understand.

Column-Level Permissions

DBAs can now grant permissions (SELECT, UPDATE) on specific columns as well as on entire tables. This makes it easier to secure sensitive data in your database.

Could anyone please give me the example of how to grant column level permissions? Basically, I want to give permissions to set of users(user-group) to only couple of columns in my table.

Thanks,
Dipti