Hi steef janssen,
I don’t have experience on this deployment. From my opinion, basically you can integrate SSRS 2008 with SharePoint Portal Server 2003. However you can only use SharePoint web parts such as SharePoint Report Explorer and Viewer web parts for users to view reports from SharePoint. You can’t use SharePoint integrated mode deployment. The web parts are good enough for simple report-viewing needs however they don’t provide the deep security and database-level integration with SharePoint or the report-management experience via SharePoint which are enabled with SharePoint integrated mode.
For more information, please refer to:
http://msdn.microsoft.com/en-us/library/bb326405.aspx
Hope this helps.
Lu Zou