There are already too many links around, please either keep them valid or auto-replace them when the beta's successfully over.
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The sitename.stackexchange.com name will always work.
When a site leaves beta (and received it's final name/url), we will set up a 301 permanent redirect to the "real" name (e.g. Nothingtoinstall.com for Webapps). Both domain names will work, even after the beta, and we will not lose the "Google Juice" when we rename the site.
There are lots of people who panic that their blog posts, tweets, etc will become invalid when the new domain name is added. That is not the case.
I think this is highly likely anyway, at least for quite some time after the site goes gold. All that's needed is a "301 Moved Permanently" HTTP response with the new URL. It wouldn't be too difficult go through the database replacing old links with new ones either.