The migration is pretty straight forward when moving from SSRS 2005 to SSRS 2008. Just backup the ReportServer and ReportServerTempdb from 2005 and restore them to 2008, then point RS 2008 at them. RS 2008 will migrate the schema for you. You will need to create the RSExec role first and don't forget to backup and then restore the encryption key. See the link below for some considerations.

http://msdn.microsoft.com/en-us/library/ms143747.aspx

One final note that got me; make sure your data sources do not have spaces in them prior to upgrading from 2005 to 2008. RS 2008 will replace the spaces with an underscore and all reports depending on the data source will break. It's best to fix this prior to the migration.