Like Scott said, it's like driving a Ford to your job at Toyota. <grin/>
If you're using Spring properly (for Transactions, Hibernate integration, and so on), you could easily deploy your application on Tomcat. Spring does transactions, so you don't need Stateless Session Beans anymore. The Hibernate integration is fabulous.
If you need JMS messaging, you could use ActiveMQ. Jencks (a related project) allows you to add Message-Driven POJOs to your Spring environment, so you don't need MDBs anymore, either. Again, you could put this on Tomcat.