Apache's mod_rewrite can be used to do time-sensitive redirects... handy if you have to make a scheduled change at an inconvenient time. But even better, what if you need to get to the original site? This example also includes a url /backdoor that sets a 15 min cookie, redirects to the main page, and an exclusion to not redirect anyone who has that cookie set. Cool stuff.
RewriteEngine On
# Start redirecting after this datetime
RewriteCond %%%%% >200904040900
# Don't redirect certain paths
RewriteCond % !^/favicon.ico
RewriteCond % !^/webservices
RewriteCond % !^/backdoor
# Don't redirect if backdoor cookie is active
RewriteCond % !backdoor
# Do the rewrite
RewriteRule .* http://mynewhostname/ [R,L]
# Allow back door access to old site (this site) - hit /backdoor and they get a cookie for
# 15 mins such that they won't be redirected while it is active.
RewriteRule ^/backdoor http://myoldhostname/ [CO=backdoor:yes:myoldhostname:15:/]