Also not that MyFaces has achieved 100% compliance of JSF 1.2, as announced here:
So perhaps the Sun RI 1.2 change will not be necessary now?
The current documentation is for Seam 1.2. You should use the JEMS 4.0 installer for it. Seam 1.3 targets JBoss 4.2.
Going forward (Seam 1.3) we're still going to recommend using the JSF RI. I hope that the new MyFaces will be a viable platform, but I don't think we've done any testing at all with it.
The seam doco (getting started etc.) still recommends using the JEMS installer.