From my experience there is no great difference functionally testing a J2EE application from any other client server type application (if testing from a client perspective).
Where things may be a little different is when you come to do performance and load testing. Depending on your project requirements and on your implimentation it may be necessary for example to test particular servlets, EJB's in isolation for performance purposes.
If this is not the case then even Load Testing a J2EE is not drastically different from a more traditional client server application. However it does throw more parameters into the equation though e.g. The Application Server used, Java Virtual Machine etc...
If you need to functionally test the server side of your J2EE application you may need a tool such as JUNIT or HTTPUnit to aide with this.
Hope this helps.
[This message has been edited by coling (edited 01-21-2003).]