ANSI_NULL property identifies how NULL values are evaluated by SQL Server. Basically query behavior changes with this value set.
ANSI SQL-92 standard basically states that when comparing two null values, the result will be FALSE. SQL Server can be configured to evaluate this condition to be TRUE with this setting turned off.
ANSI SQL-92: NULL NULL
Non ANSI SQL-92: NULL=NULL
More information on this can be found in SQL Server's books online articles.
With all that being said, the answer to your question specifically is - setting this database to compatability mode 80 may result in some unexpected query result sets, especially if you're changing the way NULL values are evaulated by the query engine.
Also, be aware with an indexed view, SQL Server Standard Edition handles this somewhat differently than SQL Server Enterprise. Basically the Standard edition - you have to specifiy in the query code (or by use of a view, tbf, or stored proc) to use the indexed view, while in Enterprise, SQL Server automatically uses the indexed view.
"Indexed views can be created in any edition of SQL Server. In SQL Server Enterprise, the query optimizer automatically considers the indexed view. To use an indexed view in all other editions, the NOEXPAND table hint must be used."