I'm guessing that since Solaris Express is used for development of the next version of Solaris (Solaris 11) and Solaris 11 when it comes out, is supposed to be as stable and rock solid as Solaris 9 and Solaris 10 were, they probably wanted to work out all the IPS package manager bugs in OpenSolaris 2008.05 Project Indiana before they backported it to Solaris Express.
Think of it with this Linux analogy: OpenSolaris 2008.05 Project Indiana is equivalent to "Red Hat Fedora" (i.e. community driven open source OS used to test out radical new ideas). Solaris Express is supposed to be like "Red Hat Enterprise Linux" (i.e. enterprise grade OS) except that you don't need to buy a license from Red Hat (or sun) to download Solaris 10 or Solaris Express ;-) -- This message posted from opensolaris.org
