Sorry gang, that was my fault. Forgot to update 2 composite*.xml files when I was publishing the bits.
So... the site worked (pointing at Alpha1b) until I did the usual rename from Alpha1b to Alpha1, and works again when I realized I'd forgotten to ALSO update the metadata in http://download.jboss.org/jbosstools/updates/development/juno/ to ALSO point at the Alpha1 (not Alpha1b) bits.
Mea culpa.