Serge van den Oever works for Macaw, A Dutch Systems Integrator and Microsoft Partner. At yesterdays event of the Dutch Information Worker User Group he gave a very good overview of BPOS. He also shows a good example of an application running on Windows Azure which is integrated with SharePOint Online Standard.
I uploaded the presentation at my skydrive.
The presentation starts with a general introduction to BPOS. It then zooms in on the features of SharePoint Online and gives some ideas on how you could integrate Silverlight applications with BPOS.
Part of the presentation is in Dutch, but I think most of the presentation can be understood by English readers as well because most computer related terms are in English.