Just adding to Avi's comments...
That is what the spec says (section 24.1.2 Programming restrictions)
� An enterprise Bean must not use read/write static fields. Using read-only static fields is allowed. Therefore, it is recommended that all static fields in the enterprise bean class be declared as final.
This rule is required to ensure consistent runtime semantics because while some EJB Containers may use a single JVM to execute all enterprise bean�s instances, others may distribute the instances across multiple JVMs.

Alex (SCJP 1.4, SCBCD 1.3, SCWCD 1.4, SCJD 1.4)