Google IPv6 service now added to all domains on "ghs.google.com". If your App Engine app runs on your own domain using Google Apps, refer App Engine joined Google IPv6, which discusses "caveats of parsing client IP addresses, and how to use SixXS Gateway to test site over IPv6.
"Currently, Google over IPv6 program presents AAAA records only to ISPs within our whitelist. This helps ensure that our services remain reliable, by reducing exposure to clients with broken IPv6 connectivity. If you want your domain bypass whitelist, just change your CNAME pointer from ghs.google.com to ghs46.google.com," explains Google.
Note: while ghs46 will cause your App Engine site to receive a larger fraction of its traffic over IPv6, our studies show that it may make your site slow or unreachable for up to 0.1% of clients across Internet, so use this alternate domain with care.