What it means is that they'll be advertising AAAA records (IPv6 addresses) for www.google.com
instead of just A records (IPv4 addresses). In practice this should make no difference at all for IPv4 only hosts. Where it does make a difference is when the host is configured for both IPv4 and IPv6, it will ask for any AAAA records before trying A records. Previously if you'd "accidentally" ended up asking for an AAAA record for www.google.com
you wouldn't have been given any IPv6 records. The only time you'll actually notice any difference from this as an end user is when you have mis-configured IPv6 lying around or possibly with very slow DNS.
Providers like google etc. are scared of the flak they'll take from users with mis-configured systems if they advertised both normally, hence this "test day" is an opportunity for them to see if there is a big issue and for users to see if they have IPv6 support.
Edited by awoodland (Thu 13-Jan-11 14:53:53)