I have been involved in several major J2EE projects for banks and insurance companies. In many cases, the business processes have been running for years in COBOL or PL1 or some other Host language. So setting up a J2EE environment mostly means "webifying" the existing business process and the Appserver is basically just there to do session management and serve up JSPs.
The performance of these systems has varied tremendjously. Some of them use JMS to send messages to the back end. These generally work well, becuase the processes are async anaway. But we have had mixed results with applications which do a lot of direct manipulatin of a database.
Does anyone else have xperiences with using middleware to call PL-SQL or other database intensive processes? What kind of performance/scalability did you get? Are there any good references to this subject?