Appian 6.6.1 mentions JBoss 5.1 as one of the certified webapplication servers.

JBoss EAP = JBoss Enterprise Application Platform
www.redhat.com/.../

You want to run on an application server that you have a maintenance agreement in place. Community Edition (which is an option on Appian installer) is only for non-production as that is not "maintained" by a vendor with security patches, etc.

I'm aware that you should use a supported webapplication server. We are using JBoss EWS (Enterprise Web Server) and not EAP. My question was if Appian has a hard requirement for EAP. If EWS is sufficient, then we will use this version and not the EAP version.

You need EAP, not EWS.

Do you have arguments why EAP is required ?

It is the version tested and supported to work with Appian.

The arguement is a bit simpler, we require EAP because we make use of message driven EJB and JMS in the application server. EWS doesnt support this so we it will not work. EWS is basically just Tomcat.