Can a sql 2005 database that has a web front end, utilize a SQL Server 2008 reporting services database for reporting?
If you mean using your SQL Server 2005 database as a datasource database for full implementation of SSRS 2008 the answer is yes but you will need to design and create Views and separate stored procedures so you will not create performance and deadlock issues for using same resources for both web and reporting.
The other option is to use SSIS package to populate another database and use that as your reporting datasource.