Zentyal is open source, so is Openchange. On the same note Sogo, Samba, Postfix, Dovecot and Ubuntu are all open source projects.
When Microsoft and Apple stop publishing things for years at a time on their website it's one thing, but when Openchange doesn't put anything on a website it usually means that someone or everyone is hard at work.
In fact there's talk around the water cooler that Zentyal took over Openchange development, which was being spearheaded by Sogo. Then there's the fact that Sogo's latest version seems to just have Openchange fully incorporated into it. I haven't had anyone from either project call me and tell me personally that this is all factual, but I do know for sure that Zentyal is hard at work getting ready to launch v5.xx for all of us beta testers. If that's the case then we're all going to get to play with a super cool release and from the hints I've been getting it's going to be pretty bad a**!