I have no idea about SaaS. Ask some specific questions, or list your specific requirements and I'm sure people can help.
You can have a look at Microsoft reference implementation for SaaS sample application LitewareHR at http://msdn2.microsoft.com/en-us/architecture/bb229292.aspx
In order to improve scalability ,we are redesign the application to run on MySQL ,JBoss 4.0.5 (EJB3 with clusterings).
My question would it be easy to implement single instance multi-tenant architecture based on Jboss Seam framework?
For example if the dvdstore example is multi-company application ,the workflow definition,security will be unique for each company.
I m sorry if my question is not revelant to JBoss Seam