Or you could just try doing the .htaccess trick I posted over at central ("non-www to www urls", in the faq forum); one update globally forces users to use one URL at all times, and you don't have to re-apply each time you upgrade
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What I do on my forums however, since I own numerous domains and allow SSL, is set it so people can access the board from anywhere, granted cookie issues can arise, but I figure if someone is specifially accessing the site via our .net or .org (or ssl) there's a reason for it (such as an anal retentive sysadmin at their office).
