I started learning JBPM5 a few months ago. Based on what I know so far, what is installed on the server are the following components (quoted from the user guide)
Guvnor repository - optional component that can be used to store all your business processes. It supports collaboration, versioning, etc. There is integration with both the Eclipse plugin and web-based designer, supporting round-tripping between the different tools.
jBPM console - web-based console that allows business users to manage their business processes (start new processes, inspect running instances), their task list and see reports.
Web-based Designer - tool used to model the process
From what I've seen, these are just WAR files deployed by the installer so it should theoretically be possible to deploy to other servers.
If by your question, you mean "can I use JBPM only with JBoss AS?", then the answer is no. The process engine itself is just like any other Java library and can be used by any JVM-based app - it does not even have to be a web-based application.
In our implementation for instance, we don't use the Guvnor, Console, and Designer. We just use the process engine (just a bunch of JARs) embedded in our web application. As "just another library" like any of the other Java libraries, it works well when we do unit-testing using Jetty or Tomcat when deployed.