The problem was a misleading documentation. To successfully build Seam 2.3.0 beta-2 you have to:
1) copy the file <seam-source-root>/build/ci.settings.xml to the directory <user-home>/.m2/settings.xml
2) mvn clean install
Step 1 is important but not documented.
Step 1 should be:
Copy build/settings.xml from http://anonsvn.jboss.org/repos/seam/branches/community/Seam_2_3/build/settings.xml or from Seam 2.3 distribution.
build/ci.settings.xml is Maven CI build settings specific file