1. be on T-Mobile’s unlimited data-plan. This also includes those who managed to get the $10 unlimited plan.
2. register on their website by filling out a form and wait for the confirmation. The response time in my case was a couple of hours.
The setup for the N900 is quite straightforward. In general the following will setup a dual-stack (IPv6/IPv4),
1. Install Power kernel.
2. Download the 2 enable and disable scripts,place them in the /etc/network/if-up.d/ and /etc/network/if-down.d/ folders respectively, and make them executable.
3. Edit the APN info in the 00_enable_apn_ipv6 script.
4. Enable IPv6 DNS in the browser. about:config. set disableIPv6 to false. Its worth noting that Opera Mobile supports it by default!
5. Restart 3G service using 3G/2G Mode Selection Applet. Select 2G then 3G.
6. Start the T-Mobile internet connection and navigate to http://ipv6.whatismyipv6.com/ to see your IPv6 address.
The Browser will work but Skype won’t. Other services not yet working (IPv6 enabled) are MSN and Tor.
There’s some useful info on Google’s IPv6 service, here