I think all announcements, or at least PAID license holders should receive an email that a new version of the software is available to download. It shouldn't be by chance of visiting here that they are made aware they're forum isn't up to date.
I was upset to see the first beta sale happened without email notice. I would have bought a forum license then. I can't imagine how many others would have done the same if they knew it was happening. How many people preordered VB4 due to an email notice the sale was going on? Honestly, if xen wants to grow by a significant amount of licenses, they should be emailing all registered users when new updates are out. If they don't want notices, they can opt out.
but one of the headers on the updates state: This is Unsupported Software
I think that says enough really, we can't expect them to build everything in when the product isn't finished yet. Technically they are selling you a car, but telling you that you shouldn't drive it - if you do drive it, it's your own risk and we won't support you driving it.
Technically, this software shouldn't be used on live sites (even though most of us are), they have no obligation to do this yet.
Emails will go out - for stable releases. Obviously pre-1.0 is a bit of a different situation, but most users shouldn't be upgrading to non-stable releases, so sending out an email isn't really the best idea. Users that are active are the more general target for non-stable releases.
The emails that go out won't be instant either. Security announcements would probably go out more quickly, but I'd expect to see a delay between a maintenance release becoming available and an email being sent out to customers.