First scenario:

• From http://download.jboss.org/jbosstools/updates/stable/helios, install only 'Maven Support->JBoss Maven Integration'
• Check out the Seam Forge sources from git://github.com/seam/forge
• Import the checked out sources as Maven projects: 'Import->Maven->Existing Maven Projects'
  => everything works fine

Second scenario:

• From http://download.jboss.org/jbosstools/updates/stable/helios, install
  + 'Maven Support->JBoss Maven Integration'
  + 'Maven Support->JBoss Maven CDI Configurator'
• Perform the two last steps from the first scenario
  => the error illustrated in the attached screenshot appears