get extension developers to mail there customers or write a news?
extend docs.joomla.org with the new Q&A site?
motivate Joomla magazines to promote the site?
The FAQ in the mail makes it clear:
Q: How can I help the site succeed?
A: The minute the public beta opens, invite your
friends. Spread the word via Twitter, blogs, and email far and wide.
Hang out on the site answering incoming questions during the first few
hours so that newcomers understand what a great experience a working,
popular Stack Exchange site can be.
Totally agree with you. I will personally be spreading the word on Facebook, LinkedIn and on my Extensions site (unfortunately I won't be mailing customers though). I assume the PLT will be dealing with the news article on the Joomla site. The thing we need to do is ensure the word is spread well and properly so that people post on the joomla.stackexchange site rather than using the Joomla tag on SO. If nobody else does it then I will update the excerpt/description for the Joomla tag adding a link to the new site
Before you jump too far down that path, I'd form a management "team" (aka working group) out of the beta participants, and have that team work out what JSE should/could be. From that plan will come how you need to market JSE and also how it may (or may not) slot into the official Joomla organisation structure.
I would be very careful about posting on the forum because this resource directly competes with it and I'm sure there are people that feel threatened by it. Whether that is justified or not doesn't really matter - it's change and some people don't like change.
I think you could promote on the Joomla magazine site but I wouldn't move too fast without a team and a plan.
We could contact well known Joomla websites and ask them to make a short entry about joomla.stackexchange.com on their blogs (if they have them). I can think of a few big ones such as k2, nonumber, kunena, easydiscuss, akeeba, sourcecoast, jomsocial,cb etc etc
Perhaps ask them to link to the page that explains how to ask questions first though. People need to know this it's not a discussion forum....but then again it might discourage people :X