also notice it says something like "At certain times, Red Hat may extend errata maintenance for certain popular releases of the operating system."
i'm guessing that 8 will be supported for quite a while, and i really couldn't be convinced for sure that that was part of RHN or not, as long as you pay the whatever a year, you should be able to get security fixes for it, if not that is pretty lame, especially for RH8, which hasn't been out for very long