As the European RIPE is running out of IPv4 address space, the US government is actually requiring its own websites to be accessible through IPv6. Of course, major websites such as Google and Facebook have been accessible through IPv6 for a long time.
Interestingly whitehouse.gov is using round-robin DNS to serve the website from various locations world-wide include Europe and Japan. This differs from www.whitehouse.gov which is handled by Akamai.
The US government is mandating the use of IPv6 for its own web-sites. (Slashdot)
US government paves the way to IPv6 with mandate compliance (Enterprisenetworkingplanet.com)