I don't know if you want to use full ejb, or only the persistency part (JPA) We have experience with OpenJpa in Tomcat with Spring and basically you get a Java client type of j2ee support. Simple advice: stick to Seam/Jboss. We lost precious time in finding out how to configure, load , test etc. After all that effort the next question was about standards, extensibility etc. A simple estimate is that it will take between one and two weeks to get its basics working,
The Hibernate JPA implementation is in one word great, has a number of additional features such as filtering and full text serach and has DB-auditing
around the corner(ENVERS).
Hibernate and Seam is a perfect match and doesn't need any additional tuning. JBoss5 uses the TomCat container for the web, so why complicate your life.