Are you sure you need to convert it externally? I've taken databases from SQL 2000 and SQL 2005 and copied them over to a SQL 2008 database and the database itself does the conversion. I've done this, literally, hundreds of times, and only come accross a snag twice. The problem is, once it's converted to SQL 2008 you can't convert back.
There is no need to convert it. MSSQL 2005 databases can be attached to 2008 and run in a backwards compatibility mode. This is not entirely true. You can use the "Generate SQL Server Scripts Wizard" to export the data to a MSSQL 2005 compatible .sql script.