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home: http://www.google.com/a/

Configuring Sites redirection with mod_proxy

(Apache 2.0)
    ServerName www.yourdomain.com
    DocumentRoot "/yourdomain.com/www/htdocs"
    
    RewriteEngine on
    RewriteRule ^/$ /public/ [R]
    RewriteRule ^/a/yourdomain.com/public/(.*) /public/$1 [R]
    
    ProxyRequests On
    ProxyPreserveHost Off
    SetEnv force-proxy-request-1.0 1
    ProxyPass /public/ http://sites.google.com/a/yourdomain.com/public/
    ProxyPassReverse /public/ http://sites.google.com/a/yourdomain.com/public/

this will allow you to display the google site public on those URL
http://www.yourdomain.com/
http://www.yourdomain.com/public
and keep your domain in the location bar

but there are few catches, it will not keep you signed in and as soon
as you'll click sign in at the bottom of the page you will end up again
on the URL htttp://sites.google.com/a/yourdomain.com/public

update:
you will not need to do that anymore
see: http://googlesitesblog.blogspot.com/2008/07/google-sites-on-your-domain.html