EAP 5 Adminisration and Configuration Guide 15.2. Datasource Parameters <prefill> default value in not true, but the documents said default is true.

The following Test can support my point of view
1. install JBoss EAP 5.1.2
2. deploy oracle-ds.xml, with <prefill>true</prefill>, check from jmx-console and admin-console, jboss.jca:name=XXXX,service=ManagedConnectionPool prefill value is true, the minimum number of connections has been created to connections pool
3. deploy oracle-ds.xml, with <prefill/> or no prefill confiduration, check from jmx-console and admin-console, jboss.jca:name=XXXX,service=ManagedConnectionPool prefill value is false, no connection has been created to connections pool

http://docs.redhat.com/docs/en-US/JBoss_Enterprise_Application_Platform/5/html/Administration_And_Configuration_Guide/ch15s02.html

Extract from above link:
"Whether to attempt to prefill the connection pool to the minimum number of connections. Only supporting pools (OnePool) support this feature. A warning is logged if the pool does not support prefilling. Defaults to TRUE."