As said, WebSphere 7 is a Servlet 2.5 container and thus doesn't ship with EL 2.2, but with EL 2.1. Your best bet is to install an EL 2.1 implementation which supports the same enhancements (invoking methods with arguments) as in EL 2.2. There's only one: JBoss EL. To install it, just drop jboss-el.jar in webapp's /WEB-INF/lib and add the following context parameter to your web.xml to tell MyFaces to use it instead.
EL 2.2 is not part of JSF 2.0. It's part of Servlet 3.0 which in turn is part of Java EE 6. But WebSphere 7 is a Java EE 5 container, not a Java EE 6 one. JSF 2.0 is part of Java EE 6 but backwards compatible with Servlet 2.5 / Java EE 5, that's probably where your confusion is coming from.
Thanks a lot, it's working... and even better, I'm less confused about this, thanks to you ;)