Hi Thierry
You are correct in stating that cloud is not nearly as complex as CRM OP and there is no plan to build in that complexity. For customers that require both you will need to look at what really makes sense. I think cloud now supports some custom transaction type.
The way is works is that transactions such as leads, activities, opportunities are mapped to transaction types in CRM. You can change this in the code list mapping. If you need to do something complex like determination rules on which transaction type to use this could be added to the middleware, or you could do something OP for this.
Base integration to CRM OP is possible, but if you have really complex CRM OP processes, you need to look at who really are the cloud users and what to they need. At ASUG this week Pall corporation said they have only outside sales using C4C, inside use CRM OP. Most of their outside sales reps use iPads so everything is very simple for them, complexity is hidden from them. Lennox also did a session, they also integrate with CRM. They said to start by crawling, walking, running - so start the cloud with the basic info your sales reps need and slowly add more over time.
We have a deck here that discusses our standard scenarios: http://scn.sap.com/docs/DOC-55610
I hope this helps some.
-ginger
