Which server, version, and version of seam are you using?
I am using Java 1.6.24 (there was an odd problem with a later version and weld), Jboss AS 6.1, Seam 3.1, and CDI/Weld 1.1. I also have Seam Faces and Seam Security installed - and ofc JSF 2.0
It is in fact possible to access this variable the following ways:
@Inject FacesContext fc;
user = fc.getExternalContext().getRemoteUser();