I've a similar problem but with SQL Server "auto numbering" identity fields.
I'll be facing your problem in the next few weeks as we are due to start a port to Oracle.
If you find an answer can youy post it to the list.
In jbosscmp-jdbc.xml you have to put in something like this for every entity which has a field filled by a sequence:
<entity> .... <entity-command name="oracle-sequence"> <attribute name="sequence">afsta_seq</attribute> </entity-command> </entity>