First off, it's just the basic JDK sources, no plugin or installer bundling logic has been included yet. Also the special version of Motif is not included for Linux builds, but only some of the include files are needed and they can easily be downloaded from various locations or even installed via official Linux distributions.
Second, there are a few small pieces that we still have legal issues with. So you'll see mention of "Binary Plugs" in the build process. This means that we have provided you the binary versions for these pieces, and they will be copied into your OpenJDK build during the build process. Note that you need the correct "Binary Plug" bundles from www.openjdk.org's Download area.
Fourth, ... I'm sure I forgot something... I'll re-edit this post when I remember. :^(
P.S. Windows builds will not work at this time.
P.P.S. On Solaris you need the Sun Studio 11 compilers, but if you build on Solaris Express, the Sun Studio 11 compilers are already installed.