Index on nullable column

As per http://www.ixora.com.au/tips/not_null.htm,

"An index on a nullable column cannot be used to drive access to a table unless the query contains one or more predicates against that column that exclude null values"

Though based on my test, index on a nullable column is used to access a table. Can someone clarify?

DB: 9.2.0.3
OS: W2K AS

Thanks.

• Jack