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Yep, I agree a little green light for up to date and a red one for update available. Maybe version numbers and changelogs too.
I guess this might be a bit of a hit on server resources for you guys. You could limit the check to once per day.
Also I recommend..
http://www.listmessenger.com/ You could run mailing lists totally seperate to Xoops.org membership (a good thing as people may not be members or have an up to date address).
You could have multiple newsletters ie...
One for new versions
One for urgent security updates
One for general whatever
You might be able to stick some advertising on there as well