Simon you should check out these links:
Build the enterprise with EJB 3, JBoss Seam, and Maven 2
Also if you have jboss tools download in eclipse, select new project wizard - JBoss Tools - project examples, select seam and PhotoAlbum EAR mavenized example. This will help understand setup seam via maven.
main points to know is your parent pom.xml must have packaging of type ear and define your plugin element
<plugin> <artifactId>maven-ear-plugin</artifactId> ..... </plugin>
I basically followed these instructions to create a Seam web project with Maven support http://docs.jboss.org/tools/3.1.0.GA/en/seam/html/create_mavenized_seam.html.
Actually, I had already created a Seam project, so I created a new one following those instructions, then I used the new projects to figure out what I needed to change (.project, .classpath, .settings/, pom.xml, etc.) to enable Maven support for my pre-existing projects. It's a bit of a pain to compare the projects, but it works...mostly. I'm still trying to figure out why my test project's tests aren't working.