More from my journey of Siebel- BIP integration discovery last week. It was a steep learning curve but none the less very useful.
Firstly, there are 3 levels of integration available to Siebel 8.1.1 users

Level 1
- Siebel stores and manages the templates and manages data generation via integration objects
- At runtime, Siebel calls BIP via WS to process data+template to generate output and returns the document to the user.
- BIP just acts as a publishing service for Siebel
Level 2
- BIP stores the templates and a report definition. The report uses complex Siebel web services as its data source.
- At runtime, BIP is called via a button or menu item (custom), it then calls Siebel back via a WS to fetch the data (integration object)
- BIP then generates output and returns the document to the Siebel UI
- This is the recommended integration as user security can be applied to the data being returned by Siebel
Level 3
- BIP stores the templates and a report definition. The report uses SQL or stored procedure calls to retrieve data from the underlying database
- At runtime, BIP is called via a button or menu item (custom), BIP fetches data from Siebel DB
- BIP generates output and returns to Siebel UI
More details in a great white paper from John on the Siebel PM team, Siebel - BI Publisher Integration Concepts
If you are looking for info on how to migrate your existing Actuate reports in the context of Siebel - check out Kan's article over at the BIP Consulting Blog Once you have downloaded the migration assistant check out the viewlet and accompanying white paper.